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3,839 | First cases of coronavirus disease 2019 (COVID-19) in the WHO European Region, 24 January to 21 February 2020
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7068164/
SHA: ce358c18aac69fc83c7b2e9a7dca4a43b0f60e2e
Authors: Spiteri, Gianfranco; Fielding, James; Diercke, Michaela; Campese, Christine; Enouf, Vincent; Gaymard, Alexandre; Bella, Antonino; Sognamiglio, Paola; Sierra Moros, Maria José; Riutort, Antonio Nicolau; Demina, Yulia V.; Mahieu, Romain; Broas, Markku; Bengnér, Malin; Buda, Silke; Schilling, Julia; Filleul, Laurent; Lepoutre, Agnès; Saura, Christine; Mailles, Alexandra; Levy-Bruhl, Daniel; Coignard, Bruno; Bernard-Stoecklin, Sibylle; Behillil, Sylvie; van der Werf, Sylvie; Valette, Martine; Lina, Bruno; Riccardo, Flavia; Nicastri, Emanuele; Casas, Inmaculada; Larrauri, Amparo; Salom Castell, Magdalena; Pozo, Francisco; Maksyutov, Rinat A.; Martin, Charlotte; Van Ranst, Marc; Bossuyt, Nathalie; Siira, Lotta; Sane, Jussi; Tegmark-Wisell, Karin; Palmérus, Maria; Broberg, Eeva K.; Beauté, Julien; Jorgensen, Pernille; Bundle, Nick; Pereyaslov, Dmitriy; Adlhoch, Cornelia; Pukkila, Jukka; Pebody, Richard; Olsen, Sonja; Ciancio, Bruno Christian
Date: 2020-03-05
DOI: 10.2807/1560-7917.es.2020.25.9.2000178
License: cc-by
Abstract: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters’ index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
Text: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters' index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
A cluster of pneumonia of unknown origin was identified in Wuhan, China, in December 2019 [1] . On 12 January 2020, Chinese authorities shared the sequence of a novel coronavirus termed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) isolated from some clustered cases [2] . Since then, the disease caused by SARS-CoV-2 has been named coronavirus disease 2019 (COVID -19) . As at 21 February 2020, the virus had spread rapidly mostly within China but also to 28 other countries, including in the World Health Organization (WHO) European Region [3] [4] [5] .
Here we describe the epidemiology of the first cases of COVID-19 in this region, excluding cases reported in the United Kingdom (UK), as at 21 February 2020. The study includes a comparison between cases detected among travellers from China and cases whose infection was acquired due to subsequent local transmission.
On 27 January 2020, the European Centre for Disease Prevention and Control (ECDC) and the WHO Regional Office for Europe asked countries to complete a WHO standard COVID-19 case report form for all confirmed and probable cases according to WHO criteria [6] [7] [8] . The overall aim of surveillance at this time was to support the global strategy of containment of COVID-19 with rapid identification and follow-up of cases linked to affected countries in order to minimise onward transmission. The surveillance objectives were to: describe the key epidemiological and clinical characteristics of COVID-19 cases detected in Europe; inform country preparedness; and improve further case detection and management. Data collected included demographics, history of recent travel to affected areas, close contact with a probable or confirmed COVID-19 case, underlying conditions, signs and symptoms of disease at onset, type of specimens from which the virus was detected, and clinical outcome. The WHO case definition was adopted for surveillance: a confirmed case was a person with laboratory confirmation of SARS-CoV-2 infection (ECDC recommended two separate SARS-CoV-2 RT-PCR tests), irrespective of clinical signs and symptoms, whereas a probable case was a suspect case for whom testing for SARS-CoV-2 was inconclusive or positive using a pan-coronavirus assay [8] . By 31 January 2020, 47 laboratories in 31 countries, including 38 laboratories in 24 European Union and European Economic Area (EU/EEA) countries, had diagnostic capability for SARS-CoV-2 available (close to 60% of countries in the WHO European Region), with cross-border shipment arrangements in place for many of those lacking domestic testing capacity. The remaining six EU/EEA countries were expected to have diagnostic testing available by mid-February [9] .
As at 09:00 on 21 February 2020, 47 confirmed cases of COVID-19 were reported in the WHO European Region and one of these cases had died [4] . Data on 38 of these cases (i.e. all except the nine reported in the UK) are included in this analysis.
The first three cases detected were reported in France on 24 January 2020 and had onset of symptoms on 17, 19 and 23 January respectively [10] . The first death was reported on 15 February in France. As at 21 February, nine countries had reported cases ( Figure) : Belgium (1), Finland (1), France (12), Germany (16), Italy (3), Russia (2), Spain (2), Sweden (1) and the UK (9 -not included further).
The place of infection (assessed at national level based on an incubation period presumed to be up to 14 days [11] , travel history and contact with probable or confirmed cases as per the case definition) was reported for 35 cases (missing for three cases), of whom 14 were infected in China (Hubei province: 10 cases; Shandong province: one case; province not reported for three cases). The remaining 21 cases were infected in Europe. Of these, 14 were linked to a cluster in Bavaria, Germany, and seven to a cluster in Haute-Savoie, France [12, 13] . Cases from the Bavarian cluster were reported from Germany and Spain, whereas cases from the Haute-Savoie cluster were reported from France All but two cases were hospitalised (35 of 37 where information on hospitalisation was reported), although it is likely that most were hospitalised to isolate the person rather than because of severe disease. The time from onset of symptoms to hospitalisation (and isolation) ranged between 0 and 10 days with a mean of 3.7 days (reported for 29 cases). The mean number of days to hospitalisation was 2.5 days for cases imported from China, but 4.6 days for those infected in Europe. This was mostly a result of delays in identifying the index cases of the two clusters in France and Germany. In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six took only a mean of 2 days to be hospitalised.
Symptoms at the point of diagnosis were reported for 31 cases. Two cases were asymptomatic and remained so until tested negative. The asymptomatic cases were tested as part of screening following repatriation and during contact tracing respectively. Of the remaining 29, 20 reported fever, 14 reported cough and eight reported weakness. Additional symptoms reported included headaches (6 cases), sore throat (2), rhinorrhoea (2), shortness of breath (2), myalgia (1), diarrhoea (1) and nausea (1). Fever was reported as the sole symptom for nine cases. In 16 of 29 symptomatic cases, the symptoms at diagnosis were consistent with the case definition for acute respiratory infection [16] , although it is possible that cases presented additional symptoms after diagnosis and these were not reported.
Data on pre-existing conditions were reported for seven cases; five had no pre-existing conditions while one was reported to be obese and one had pre-existing cardiac disease. No data on clinical signs e.g. dyspnea etc. were reported for any of the 38 cases.
All hospitalised cases had a benign clinical evolution except four, two reported in Italy and two reported in France, all of whom developed viral pneumonia. All three cases who were aged 65 years or over were admitted to intensive care and required respiratory support and one French case died. The case who died was hospitalised for 21 days and required intensive care and mechanical ventilation for 19 days. The duration of hospitalisation was reported for 16 cases with a median of 13 days (range: 8-23 days). As at 21 February 2020, four cases were still hospitalised.
All cases were confirmed according to specific assays targeting at least two separate genes (envelope (E) gene as a screening test and RNA-dependent RNA polymerase (RdRp) gene or nucleoprotein (N) gene for confirmation) [8, 17] . The specimen types tested were reported for 27 cases: 15 had positive nasopharyngeal swabs, nine had positive throat swabs, three cases had positive sputum, two had a positive nasal swab, one case had a positive nasopharyngeal aspirate and one a positive endotracheal aspirate.
As at 09:00 on 21 February, few COVID-19 cases had been detected in Europe compared with Asia. However the situation is rapidly developing, with a large outbreak recently identified in northern Italy, with transmission in several municipalities and at least two deaths [18] . As at 5 March 2020, there are 4,250 cases including 113 deaths reported among 38 countries in the WHO European region [19] .
In our analysis of early cases, we observed transmission in two broad contexts: sporadic cases among travellers from China (14 cases) and cases who acquired infection due to subsequent local transmission in Europe (21 cases). Our analysis shows that the time from symptom onset to hospitalisation/case isolation was about 3 days longer for locally acquired cases than for imported cases. People returning from affected areas are likely to have a low threshold to seek care and be tested when symptomatic, however delays in identifying the index cases of the two clusters in France and Germany meant that locally acquired cases took longer to be detected and isolated. Once the exposure is determined and contacts identified and quarantined (171 contacts in France and 200 in Germany for the clusters in Haute-Savoie and Bavaria, respectively), further cases are likely to be rapidly detected and isolated when they develop symptoms [15, 20] . In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six were hospitalised after a mean of 2 days. Locally acquired cases require significant resources for contact tracing and quarantine, and countries should be prepared to allocate considerable public health resources during the containment phase, should local clusters emerge in their population. In addition, prompt sharing of information on cases and contacts through international notification systems such as the International Health Regulations (IHR) mechanism and the European Commission's European Early Warning and Response System is essential to contain international spread of infection.
All of the imported cases had a history of travel to China. This was consistent with the epidemiological situation in Asia, and supported the recommendation for testing of suspected cases with travel history to China and potentially other areas of presumed ongoing community transmission. The situation has evolved rapidly since then, however, and the number of countries reporting COVID-19 transmission increased rapidly, notably with a large outbreak in northern Italy with 3,089 cases reported as at 5 March [18, 19] . Testing of suspected cases based on geographical risk of importation needs to be complemented with additional approaches to ensure early detection of local circulation of COVID-19, including through testing of severe acute respiratory infections in hospitals irrespectively of travel history as recommended in the WHO case definition updated on 27 February 2020 [21] .
The clinical presentation observed in the cases in Europe is that of an acute respiratory infection. However, of the 31 cases with information on symptoms, 20 cases presented with fever and nine cases presented only with fever and no other symptoms. These findings, which are consistent with other published case series, have prompted ECDC to include fever among several clinical signs or symptoms indicative for the suspected case definition.
Three cases were aged 65 years or over. All required admission to intensive care and were tourists (imported cases). These findings could reflect the average older age of the tourist population compared with the local contacts exposed to infection in Europe and do not allow us to draw any conclusion on the proportion of severe cases that we could expect in the general population of Europe. Despite this, the finding of older individuals being at higher risk of a severe clinical course is consistent with the evidence from Chinese case series published so far although the majority of infections in China have been mild [22, 23] .
This preliminary analysis is based on the first reported cases of COVID-19 cases in the WHO European Region. Given the small sample size, and limited completeness for some variables, all the results presented should be interpreted with caution.
With increasing numbers of cases in Europe, data from surveillance and investigations in the region can build on the evidence from countries in Asia experiencing more widespread transmission particularly on disease spectrum and the proportion of infections with severe outcome [22] . Understanding the infection-severity is critical to help plan for the impact on the healthcare system and the wider population. Serological studies are vital to understand the proportion of cases who are asymptomatic. Hospital-based surveillance could help estimate the incidence of severe cases and identify risk factors for severity and death. Established hospital surveillance systems that are in place for influenza and other diseases in Europe may be expanded for this purpose. In addition, a number of countries in Europe are adapting and, in some cases, already using existing sentinel primary care based surveillance systems for influenza to detect community transmission of SARS-CoV-2. This approach will be used globally to help identify evidence of widespread community transmission and, should the virus spread and containment no longer be deemed feasible, to monitor intensity of disease transmission, trends and its geographical spread.
Additional research is needed to complement surveillance data to build knowledge on the infectious period, modes of transmission, basic and effective reproduction numbers, and effectiveness of prevention and case management options also in settings outside of China. Such special studies are being conducted globally, including a cohort study on citizens repatriated from China to Europe, with the aim to extrapolate disease incidence and risk factors for infection in areas with community transmission. Countries together with ECDC and WHO, should use all opportunities to address these questions in a coordinated fashion at the European and global level.
provided input to the outline, multiple versions of the manuscript and gave approval to the final draft. | What testing and detection are needed? | {
"answer_start": [
11773
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"text": [
"Testing of suspected cases based on geographical risk of importation needs to be complemented with additional approaches to ensure early detection of local circulation of COVID-19, including through testing of severe acute respiratory infections in hospitals irrespectively of travel history as recommended in the WHO case definition updated on 27 February 2020 "
]
} | false |
3,840 | First cases of coronavirus disease 2019 (COVID-19) in the WHO European Region, 24 January to 21 February 2020
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7068164/
SHA: ce358c18aac69fc83c7b2e9a7dca4a43b0f60e2e
Authors: Spiteri, Gianfranco; Fielding, James; Diercke, Michaela; Campese, Christine; Enouf, Vincent; Gaymard, Alexandre; Bella, Antonino; Sognamiglio, Paola; Sierra Moros, Maria José; Riutort, Antonio Nicolau; Demina, Yulia V.; Mahieu, Romain; Broas, Markku; Bengnér, Malin; Buda, Silke; Schilling, Julia; Filleul, Laurent; Lepoutre, Agnès; Saura, Christine; Mailles, Alexandra; Levy-Bruhl, Daniel; Coignard, Bruno; Bernard-Stoecklin, Sibylle; Behillil, Sylvie; van der Werf, Sylvie; Valette, Martine; Lina, Bruno; Riccardo, Flavia; Nicastri, Emanuele; Casas, Inmaculada; Larrauri, Amparo; Salom Castell, Magdalena; Pozo, Francisco; Maksyutov, Rinat A.; Martin, Charlotte; Van Ranst, Marc; Bossuyt, Nathalie; Siira, Lotta; Sane, Jussi; Tegmark-Wisell, Karin; Palmérus, Maria; Broberg, Eeva K.; Beauté, Julien; Jorgensen, Pernille; Bundle, Nick; Pereyaslov, Dmitriy; Adlhoch, Cornelia; Pukkila, Jukka; Pebody, Richard; Olsen, Sonja; Ciancio, Bruno Christian
Date: 2020-03-05
DOI: 10.2807/1560-7917.es.2020.25.9.2000178
License: cc-by
Abstract: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters’ index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
Text: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters' index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
A cluster of pneumonia of unknown origin was identified in Wuhan, China, in December 2019 [1] . On 12 January 2020, Chinese authorities shared the sequence of a novel coronavirus termed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) isolated from some clustered cases [2] . Since then, the disease caused by SARS-CoV-2 has been named coronavirus disease 2019 (COVID -19) . As at 21 February 2020, the virus had spread rapidly mostly within China but also to 28 other countries, including in the World Health Organization (WHO) European Region [3] [4] [5] .
Here we describe the epidemiology of the first cases of COVID-19 in this region, excluding cases reported in the United Kingdom (UK), as at 21 February 2020. The study includes a comparison between cases detected among travellers from China and cases whose infection was acquired due to subsequent local transmission.
On 27 January 2020, the European Centre for Disease Prevention and Control (ECDC) and the WHO Regional Office for Europe asked countries to complete a WHO standard COVID-19 case report form for all confirmed and probable cases according to WHO criteria [6] [7] [8] . The overall aim of surveillance at this time was to support the global strategy of containment of COVID-19 with rapid identification and follow-up of cases linked to affected countries in order to minimise onward transmission. The surveillance objectives were to: describe the key epidemiological and clinical characteristics of COVID-19 cases detected in Europe; inform country preparedness; and improve further case detection and management. Data collected included demographics, history of recent travel to affected areas, close contact with a probable or confirmed COVID-19 case, underlying conditions, signs and symptoms of disease at onset, type of specimens from which the virus was detected, and clinical outcome. The WHO case definition was adopted for surveillance: a confirmed case was a person with laboratory confirmation of SARS-CoV-2 infection (ECDC recommended two separate SARS-CoV-2 RT-PCR tests), irrespective of clinical signs and symptoms, whereas a probable case was a suspect case for whom testing for SARS-CoV-2 was inconclusive or positive using a pan-coronavirus assay [8] . By 31 January 2020, 47 laboratories in 31 countries, including 38 laboratories in 24 European Union and European Economic Area (EU/EEA) countries, had diagnostic capability for SARS-CoV-2 available (close to 60% of countries in the WHO European Region), with cross-border shipment arrangements in place for many of those lacking domestic testing capacity. The remaining six EU/EEA countries were expected to have diagnostic testing available by mid-February [9] .
As at 09:00 on 21 February 2020, 47 confirmed cases of COVID-19 were reported in the WHO European Region and one of these cases had died [4] . Data on 38 of these cases (i.e. all except the nine reported in the UK) are included in this analysis.
The first three cases detected were reported in France on 24 January 2020 and had onset of symptoms on 17, 19 and 23 January respectively [10] . The first death was reported on 15 February in France. As at 21 February, nine countries had reported cases ( Figure) : Belgium (1), Finland (1), France (12), Germany (16), Italy (3), Russia (2), Spain (2), Sweden (1) and the UK (9 -not included further).
The place of infection (assessed at national level based on an incubation period presumed to be up to 14 days [11] , travel history and contact with probable or confirmed cases as per the case definition) was reported for 35 cases (missing for three cases), of whom 14 were infected in China (Hubei province: 10 cases; Shandong province: one case; province not reported for three cases). The remaining 21 cases were infected in Europe. Of these, 14 were linked to a cluster in Bavaria, Germany, and seven to a cluster in Haute-Savoie, France [12, 13] . Cases from the Bavarian cluster were reported from Germany and Spain, whereas cases from the Haute-Savoie cluster were reported from France All but two cases were hospitalised (35 of 37 where information on hospitalisation was reported), although it is likely that most were hospitalised to isolate the person rather than because of severe disease. The time from onset of symptoms to hospitalisation (and isolation) ranged between 0 and 10 days with a mean of 3.7 days (reported for 29 cases). The mean number of days to hospitalisation was 2.5 days for cases imported from China, but 4.6 days for those infected in Europe. This was mostly a result of delays in identifying the index cases of the two clusters in France and Germany. In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six took only a mean of 2 days to be hospitalised.
Symptoms at the point of diagnosis were reported for 31 cases. Two cases were asymptomatic and remained so until tested negative. The asymptomatic cases were tested as part of screening following repatriation and during contact tracing respectively. Of the remaining 29, 20 reported fever, 14 reported cough and eight reported weakness. Additional symptoms reported included headaches (6 cases), sore throat (2), rhinorrhoea (2), shortness of breath (2), myalgia (1), diarrhoea (1) and nausea (1). Fever was reported as the sole symptom for nine cases. In 16 of 29 symptomatic cases, the symptoms at diagnosis were consistent with the case definition for acute respiratory infection [16] , although it is possible that cases presented additional symptoms after diagnosis and these were not reported.
Data on pre-existing conditions were reported for seven cases; five had no pre-existing conditions while one was reported to be obese and one had pre-existing cardiac disease. No data on clinical signs e.g. dyspnea etc. were reported for any of the 38 cases.
All hospitalised cases had a benign clinical evolution except four, two reported in Italy and two reported in France, all of whom developed viral pneumonia. All three cases who were aged 65 years or over were admitted to intensive care and required respiratory support and one French case died. The case who died was hospitalised for 21 days and required intensive care and mechanical ventilation for 19 days. The duration of hospitalisation was reported for 16 cases with a median of 13 days (range: 8-23 days). As at 21 February 2020, four cases were still hospitalised.
All cases were confirmed according to specific assays targeting at least two separate genes (envelope (E) gene as a screening test and RNA-dependent RNA polymerase (RdRp) gene or nucleoprotein (N) gene for confirmation) [8, 17] . The specimen types tested were reported for 27 cases: 15 had positive nasopharyngeal swabs, nine had positive throat swabs, three cases had positive sputum, two had a positive nasal swab, one case had a positive nasopharyngeal aspirate and one a positive endotracheal aspirate.
As at 09:00 on 21 February, few COVID-19 cases had been detected in Europe compared with Asia. However the situation is rapidly developing, with a large outbreak recently identified in northern Italy, with transmission in several municipalities and at least two deaths [18] . As at 5 March 2020, there are 4,250 cases including 113 deaths reported among 38 countries in the WHO European region [19] .
In our analysis of early cases, we observed transmission in two broad contexts: sporadic cases among travellers from China (14 cases) and cases who acquired infection due to subsequent local transmission in Europe (21 cases). Our analysis shows that the time from symptom onset to hospitalisation/case isolation was about 3 days longer for locally acquired cases than for imported cases. People returning from affected areas are likely to have a low threshold to seek care and be tested when symptomatic, however delays in identifying the index cases of the two clusters in France and Germany meant that locally acquired cases took longer to be detected and isolated. Once the exposure is determined and contacts identified and quarantined (171 contacts in France and 200 in Germany for the clusters in Haute-Savoie and Bavaria, respectively), further cases are likely to be rapidly detected and isolated when they develop symptoms [15, 20] . In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six were hospitalised after a mean of 2 days. Locally acquired cases require significant resources for contact tracing and quarantine, and countries should be prepared to allocate considerable public health resources during the containment phase, should local clusters emerge in their population. In addition, prompt sharing of information on cases and contacts through international notification systems such as the International Health Regulations (IHR) mechanism and the European Commission's European Early Warning and Response System is essential to contain international spread of infection.
All of the imported cases had a history of travel to China. This was consistent with the epidemiological situation in Asia, and supported the recommendation for testing of suspected cases with travel history to China and potentially other areas of presumed ongoing community transmission. The situation has evolved rapidly since then, however, and the number of countries reporting COVID-19 transmission increased rapidly, notably with a large outbreak in northern Italy with 3,089 cases reported as at 5 March [18, 19] . Testing of suspected cases based on geographical risk of importation needs to be complemented with additional approaches to ensure early detection of local circulation of COVID-19, including through testing of severe acute respiratory infections in hospitals irrespectively of travel history as recommended in the WHO case definition updated on 27 February 2020 [21] .
The clinical presentation observed in the cases in Europe is that of an acute respiratory infection. However, of the 31 cases with information on symptoms, 20 cases presented with fever and nine cases presented only with fever and no other symptoms. These findings, which are consistent with other published case series, have prompted ECDC to include fever among several clinical signs or symptoms indicative for the suspected case definition.
Three cases were aged 65 years or over. All required admission to intensive care and were tourists (imported cases). These findings could reflect the average older age of the tourist population compared with the local contacts exposed to infection in Europe and do not allow us to draw any conclusion on the proportion of severe cases that we could expect in the general population of Europe. Despite this, the finding of older individuals being at higher risk of a severe clinical course is consistent with the evidence from Chinese case series published so far although the majority of infections in China have been mild [22, 23] .
This preliminary analysis is based on the first reported cases of COVID-19 cases in the WHO European Region. Given the small sample size, and limited completeness for some variables, all the results presented should be interpreted with caution.
With increasing numbers of cases in Europe, data from surveillance and investigations in the region can build on the evidence from countries in Asia experiencing more widespread transmission particularly on disease spectrum and the proportion of infections with severe outcome [22] . Understanding the infection-severity is critical to help plan for the impact on the healthcare system and the wider population. Serological studies are vital to understand the proportion of cases who are asymptomatic. Hospital-based surveillance could help estimate the incidence of severe cases and identify risk factors for severity and death. Established hospital surveillance systems that are in place for influenza and other diseases in Europe may be expanded for this purpose. In addition, a number of countries in Europe are adapting and, in some cases, already using existing sentinel primary care based surveillance systems for influenza to detect community transmission of SARS-CoV-2. This approach will be used globally to help identify evidence of widespread community transmission and, should the virus spread and containment no longer be deemed feasible, to monitor intensity of disease transmission, trends and its geographical spread.
Additional research is needed to complement surveillance data to build knowledge on the infectious period, modes of transmission, basic and effective reproduction numbers, and effectiveness of prevention and case management options also in settings outside of China. Such special studies are being conducted globally, including a cohort study on citizens repatriated from China to Europe, with the aim to extrapolate disease incidence and risk factors for infection in areas with community transmission. Countries together with ECDC and WHO, should use all opportunities to address these questions in a coordinated fashion at the European and global level.
provided input to the outline, multiple versions of the manuscript and gave approval to the final draft. | What did the finding prompt ECDC to do? | {
"answer_start": [
12486
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"text": [
"include fever among several clinical signs or symptoms indicative for the suspected case definition."
]
} | false |
3,841 | First cases of coronavirus disease 2019 (COVID-19) in the WHO European Region, 24 January to 21 February 2020
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7068164/
SHA: ce358c18aac69fc83c7b2e9a7dca4a43b0f60e2e
Authors: Spiteri, Gianfranco; Fielding, James; Diercke, Michaela; Campese, Christine; Enouf, Vincent; Gaymard, Alexandre; Bella, Antonino; Sognamiglio, Paola; Sierra Moros, Maria José; Riutort, Antonio Nicolau; Demina, Yulia V.; Mahieu, Romain; Broas, Markku; Bengnér, Malin; Buda, Silke; Schilling, Julia; Filleul, Laurent; Lepoutre, Agnès; Saura, Christine; Mailles, Alexandra; Levy-Bruhl, Daniel; Coignard, Bruno; Bernard-Stoecklin, Sibylle; Behillil, Sylvie; van der Werf, Sylvie; Valette, Martine; Lina, Bruno; Riccardo, Flavia; Nicastri, Emanuele; Casas, Inmaculada; Larrauri, Amparo; Salom Castell, Magdalena; Pozo, Francisco; Maksyutov, Rinat A.; Martin, Charlotte; Van Ranst, Marc; Bossuyt, Nathalie; Siira, Lotta; Sane, Jussi; Tegmark-Wisell, Karin; Palmérus, Maria; Broberg, Eeva K.; Beauté, Julien; Jorgensen, Pernille; Bundle, Nick; Pereyaslov, Dmitriy; Adlhoch, Cornelia; Pukkila, Jukka; Pebody, Richard; Olsen, Sonja; Ciancio, Bruno Christian
Date: 2020-03-05
DOI: 10.2807/1560-7917.es.2020.25.9.2000178
License: cc-by
Abstract: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters’ index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
Text: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters' index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
A cluster of pneumonia of unknown origin was identified in Wuhan, China, in December 2019 [1] . On 12 January 2020, Chinese authorities shared the sequence of a novel coronavirus termed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) isolated from some clustered cases [2] . Since then, the disease caused by SARS-CoV-2 has been named coronavirus disease 2019 (COVID -19) . As at 21 February 2020, the virus had spread rapidly mostly within China but also to 28 other countries, including in the World Health Organization (WHO) European Region [3] [4] [5] .
Here we describe the epidemiology of the first cases of COVID-19 in this region, excluding cases reported in the United Kingdom (UK), as at 21 February 2020. The study includes a comparison between cases detected among travellers from China and cases whose infection was acquired due to subsequent local transmission.
On 27 January 2020, the European Centre for Disease Prevention and Control (ECDC) and the WHO Regional Office for Europe asked countries to complete a WHO standard COVID-19 case report form for all confirmed and probable cases according to WHO criteria [6] [7] [8] . The overall aim of surveillance at this time was to support the global strategy of containment of COVID-19 with rapid identification and follow-up of cases linked to affected countries in order to minimise onward transmission. The surveillance objectives were to: describe the key epidemiological and clinical characteristics of COVID-19 cases detected in Europe; inform country preparedness; and improve further case detection and management. Data collected included demographics, history of recent travel to affected areas, close contact with a probable or confirmed COVID-19 case, underlying conditions, signs and symptoms of disease at onset, type of specimens from which the virus was detected, and clinical outcome. The WHO case definition was adopted for surveillance: a confirmed case was a person with laboratory confirmation of SARS-CoV-2 infection (ECDC recommended two separate SARS-CoV-2 RT-PCR tests), irrespective of clinical signs and symptoms, whereas a probable case was a suspect case for whom testing for SARS-CoV-2 was inconclusive or positive using a pan-coronavirus assay [8] . By 31 January 2020, 47 laboratories in 31 countries, including 38 laboratories in 24 European Union and European Economic Area (EU/EEA) countries, had diagnostic capability for SARS-CoV-2 available (close to 60% of countries in the WHO European Region), with cross-border shipment arrangements in place for many of those lacking domestic testing capacity. The remaining six EU/EEA countries were expected to have diagnostic testing available by mid-February [9] .
As at 09:00 on 21 February 2020, 47 confirmed cases of COVID-19 were reported in the WHO European Region and one of these cases had died [4] . Data on 38 of these cases (i.e. all except the nine reported in the UK) are included in this analysis.
The first three cases detected were reported in France on 24 January 2020 and had onset of symptoms on 17, 19 and 23 January respectively [10] . The first death was reported on 15 February in France. As at 21 February, nine countries had reported cases ( Figure) : Belgium (1), Finland (1), France (12), Germany (16), Italy (3), Russia (2), Spain (2), Sweden (1) and the UK (9 -not included further).
The place of infection (assessed at national level based on an incubation period presumed to be up to 14 days [11] , travel history and contact with probable or confirmed cases as per the case definition) was reported for 35 cases (missing for three cases), of whom 14 were infected in China (Hubei province: 10 cases; Shandong province: one case; province not reported for three cases). The remaining 21 cases were infected in Europe. Of these, 14 were linked to a cluster in Bavaria, Germany, and seven to a cluster in Haute-Savoie, France [12, 13] . Cases from the Bavarian cluster were reported from Germany and Spain, whereas cases from the Haute-Savoie cluster were reported from France All but two cases were hospitalised (35 of 37 where information on hospitalisation was reported), although it is likely that most were hospitalised to isolate the person rather than because of severe disease. The time from onset of symptoms to hospitalisation (and isolation) ranged between 0 and 10 days with a mean of 3.7 days (reported for 29 cases). The mean number of days to hospitalisation was 2.5 days for cases imported from China, but 4.6 days for those infected in Europe. This was mostly a result of delays in identifying the index cases of the two clusters in France and Germany. In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six took only a mean of 2 days to be hospitalised.
Symptoms at the point of diagnosis were reported for 31 cases. Two cases were asymptomatic and remained so until tested negative. The asymptomatic cases were tested as part of screening following repatriation and during contact tracing respectively. Of the remaining 29, 20 reported fever, 14 reported cough and eight reported weakness. Additional symptoms reported included headaches (6 cases), sore throat (2), rhinorrhoea (2), shortness of breath (2), myalgia (1), diarrhoea (1) and nausea (1). Fever was reported as the sole symptom for nine cases. In 16 of 29 symptomatic cases, the symptoms at diagnosis were consistent with the case definition for acute respiratory infection [16] , although it is possible that cases presented additional symptoms after diagnosis and these were not reported.
Data on pre-existing conditions were reported for seven cases; five had no pre-existing conditions while one was reported to be obese and one had pre-existing cardiac disease. No data on clinical signs e.g. dyspnea etc. were reported for any of the 38 cases.
All hospitalised cases had a benign clinical evolution except four, two reported in Italy and two reported in France, all of whom developed viral pneumonia. All three cases who were aged 65 years or over were admitted to intensive care and required respiratory support and one French case died. The case who died was hospitalised for 21 days and required intensive care and mechanical ventilation for 19 days. The duration of hospitalisation was reported for 16 cases with a median of 13 days (range: 8-23 days). As at 21 February 2020, four cases were still hospitalised.
All cases were confirmed according to specific assays targeting at least two separate genes (envelope (E) gene as a screening test and RNA-dependent RNA polymerase (RdRp) gene or nucleoprotein (N) gene for confirmation) [8, 17] . The specimen types tested were reported for 27 cases: 15 had positive nasopharyngeal swabs, nine had positive throat swabs, three cases had positive sputum, two had a positive nasal swab, one case had a positive nasopharyngeal aspirate and one a positive endotracheal aspirate.
As at 09:00 on 21 February, few COVID-19 cases had been detected in Europe compared with Asia. However the situation is rapidly developing, with a large outbreak recently identified in northern Italy, with transmission in several municipalities and at least two deaths [18] . As at 5 March 2020, there are 4,250 cases including 113 deaths reported among 38 countries in the WHO European region [19] .
In our analysis of early cases, we observed transmission in two broad contexts: sporadic cases among travellers from China (14 cases) and cases who acquired infection due to subsequent local transmission in Europe (21 cases). Our analysis shows that the time from symptom onset to hospitalisation/case isolation was about 3 days longer for locally acquired cases than for imported cases. People returning from affected areas are likely to have a low threshold to seek care and be tested when symptomatic, however delays in identifying the index cases of the two clusters in France and Germany meant that locally acquired cases took longer to be detected and isolated. Once the exposure is determined and contacts identified and quarantined (171 contacts in France and 200 in Germany for the clusters in Haute-Savoie and Bavaria, respectively), further cases are likely to be rapidly detected and isolated when they develop symptoms [15, 20] . In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six were hospitalised after a mean of 2 days. Locally acquired cases require significant resources for contact tracing and quarantine, and countries should be prepared to allocate considerable public health resources during the containment phase, should local clusters emerge in their population. In addition, prompt sharing of information on cases and contacts through international notification systems such as the International Health Regulations (IHR) mechanism and the European Commission's European Early Warning and Response System is essential to contain international spread of infection.
All of the imported cases had a history of travel to China. This was consistent with the epidemiological situation in Asia, and supported the recommendation for testing of suspected cases with travel history to China and potentially other areas of presumed ongoing community transmission. The situation has evolved rapidly since then, however, and the number of countries reporting COVID-19 transmission increased rapidly, notably with a large outbreak in northern Italy with 3,089 cases reported as at 5 March [18, 19] . Testing of suspected cases based on geographical risk of importation needs to be complemented with additional approaches to ensure early detection of local circulation of COVID-19, including through testing of severe acute respiratory infections in hospitals irrespectively of travel history as recommended in the WHO case definition updated on 27 February 2020 [21] .
The clinical presentation observed in the cases in Europe is that of an acute respiratory infection. However, of the 31 cases with information on symptoms, 20 cases presented with fever and nine cases presented only with fever and no other symptoms. These findings, which are consistent with other published case series, have prompted ECDC to include fever among several clinical signs or symptoms indicative for the suspected case definition.
Three cases were aged 65 years or over. All required admission to intensive care and were tourists (imported cases). These findings could reflect the average older age of the tourist population compared with the local contacts exposed to infection in Europe and do not allow us to draw any conclusion on the proportion of severe cases that we could expect in the general population of Europe. Despite this, the finding of older individuals being at higher risk of a severe clinical course is consistent with the evidence from Chinese case series published so far although the majority of infections in China have been mild [22, 23] .
This preliminary analysis is based on the first reported cases of COVID-19 cases in the WHO European Region. Given the small sample size, and limited completeness for some variables, all the results presented should be interpreted with caution.
With increasing numbers of cases in Europe, data from surveillance and investigations in the region can build on the evidence from countries in Asia experiencing more widespread transmission particularly on disease spectrum and the proportion of infections with severe outcome [22] . Understanding the infection-severity is critical to help plan for the impact on the healthcare system and the wider population. Serological studies are vital to understand the proportion of cases who are asymptomatic. Hospital-based surveillance could help estimate the incidence of severe cases and identify risk factors for severity and death. Established hospital surveillance systems that are in place for influenza and other diseases in Europe may be expanded for this purpose. In addition, a number of countries in Europe are adapting and, in some cases, already using existing sentinel primary care based surveillance systems for influenza to detect community transmission of SARS-CoV-2. This approach will be used globally to help identify evidence of widespread community transmission and, should the virus spread and containment no longer be deemed feasible, to monitor intensity of disease transmission, trends and its geographical spread.
Additional research is needed to complement surveillance data to build knowledge on the infectious period, modes of transmission, basic and effective reproduction numbers, and effectiveness of prevention and case management options also in settings outside of China. Such special studies are being conducted globally, including a cohort study on citizens repatriated from China to Europe, with the aim to extrapolate disease incidence and risk factors for infection in areas with community transmission. Countries together with ECDC and WHO, should use all opportunities to address these questions in a coordinated fashion at the European and global level.
provided input to the outline, multiple versions of the manuscript and gave approval to the final draft. | Why is understanding the infection-severity critical ? | {
"answer_start": [
13802
],
"text": [
"to help plan for the impact on the healthcare system and the wider population."
]
} | false |
3,842 | First cases of coronavirus disease 2019 (COVID-19) in the WHO European Region, 24 January to 21 February 2020
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7068164/
SHA: ce358c18aac69fc83c7b2e9a7dca4a43b0f60e2e
Authors: Spiteri, Gianfranco; Fielding, James; Diercke, Michaela; Campese, Christine; Enouf, Vincent; Gaymard, Alexandre; Bella, Antonino; Sognamiglio, Paola; Sierra Moros, Maria José; Riutort, Antonio Nicolau; Demina, Yulia V.; Mahieu, Romain; Broas, Markku; Bengnér, Malin; Buda, Silke; Schilling, Julia; Filleul, Laurent; Lepoutre, Agnès; Saura, Christine; Mailles, Alexandra; Levy-Bruhl, Daniel; Coignard, Bruno; Bernard-Stoecklin, Sibylle; Behillil, Sylvie; van der Werf, Sylvie; Valette, Martine; Lina, Bruno; Riccardo, Flavia; Nicastri, Emanuele; Casas, Inmaculada; Larrauri, Amparo; Salom Castell, Magdalena; Pozo, Francisco; Maksyutov, Rinat A.; Martin, Charlotte; Van Ranst, Marc; Bossuyt, Nathalie; Siira, Lotta; Sane, Jussi; Tegmark-Wisell, Karin; Palmérus, Maria; Broberg, Eeva K.; Beauté, Julien; Jorgensen, Pernille; Bundle, Nick; Pereyaslov, Dmitriy; Adlhoch, Cornelia; Pukkila, Jukka; Pebody, Richard; Olsen, Sonja; Ciancio, Bruno Christian
Date: 2020-03-05
DOI: 10.2807/1560-7917.es.2020.25.9.2000178
License: cc-by
Abstract: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters’ index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
Text: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters' index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
A cluster of pneumonia of unknown origin was identified in Wuhan, China, in December 2019 [1] . On 12 January 2020, Chinese authorities shared the sequence of a novel coronavirus termed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) isolated from some clustered cases [2] . Since then, the disease caused by SARS-CoV-2 has been named coronavirus disease 2019 (COVID -19) . As at 21 February 2020, the virus had spread rapidly mostly within China but also to 28 other countries, including in the World Health Organization (WHO) European Region [3] [4] [5] .
Here we describe the epidemiology of the first cases of COVID-19 in this region, excluding cases reported in the United Kingdom (UK), as at 21 February 2020. The study includes a comparison between cases detected among travellers from China and cases whose infection was acquired due to subsequent local transmission.
On 27 January 2020, the European Centre for Disease Prevention and Control (ECDC) and the WHO Regional Office for Europe asked countries to complete a WHO standard COVID-19 case report form for all confirmed and probable cases according to WHO criteria [6] [7] [8] . The overall aim of surveillance at this time was to support the global strategy of containment of COVID-19 with rapid identification and follow-up of cases linked to affected countries in order to minimise onward transmission. The surveillance objectives were to: describe the key epidemiological and clinical characteristics of COVID-19 cases detected in Europe; inform country preparedness; and improve further case detection and management. Data collected included demographics, history of recent travel to affected areas, close contact with a probable or confirmed COVID-19 case, underlying conditions, signs and symptoms of disease at onset, type of specimens from which the virus was detected, and clinical outcome. The WHO case definition was adopted for surveillance: a confirmed case was a person with laboratory confirmation of SARS-CoV-2 infection (ECDC recommended two separate SARS-CoV-2 RT-PCR tests), irrespective of clinical signs and symptoms, whereas a probable case was a suspect case for whom testing for SARS-CoV-2 was inconclusive or positive using a pan-coronavirus assay [8] . By 31 January 2020, 47 laboratories in 31 countries, including 38 laboratories in 24 European Union and European Economic Area (EU/EEA) countries, had diagnostic capability for SARS-CoV-2 available (close to 60% of countries in the WHO European Region), with cross-border shipment arrangements in place for many of those lacking domestic testing capacity. The remaining six EU/EEA countries were expected to have diagnostic testing available by mid-February [9] .
As at 09:00 on 21 February 2020, 47 confirmed cases of COVID-19 were reported in the WHO European Region and one of these cases had died [4] . Data on 38 of these cases (i.e. all except the nine reported in the UK) are included in this analysis.
The first three cases detected were reported in France on 24 January 2020 and had onset of symptoms on 17, 19 and 23 January respectively [10] . The first death was reported on 15 February in France. As at 21 February, nine countries had reported cases ( Figure) : Belgium (1), Finland (1), France (12), Germany (16), Italy (3), Russia (2), Spain (2), Sweden (1) and the UK (9 -not included further).
The place of infection (assessed at national level based on an incubation period presumed to be up to 14 days [11] , travel history and contact with probable or confirmed cases as per the case definition) was reported for 35 cases (missing for three cases), of whom 14 were infected in China (Hubei province: 10 cases; Shandong province: one case; province not reported for three cases). The remaining 21 cases were infected in Europe. Of these, 14 were linked to a cluster in Bavaria, Germany, and seven to a cluster in Haute-Savoie, France [12, 13] . Cases from the Bavarian cluster were reported from Germany and Spain, whereas cases from the Haute-Savoie cluster were reported from France All but two cases were hospitalised (35 of 37 where information on hospitalisation was reported), although it is likely that most were hospitalised to isolate the person rather than because of severe disease. The time from onset of symptoms to hospitalisation (and isolation) ranged between 0 and 10 days with a mean of 3.7 days (reported for 29 cases). The mean number of days to hospitalisation was 2.5 days for cases imported from China, but 4.6 days for those infected in Europe. This was mostly a result of delays in identifying the index cases of the two clusters in France and Germany. In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six took only a mean of 2 days to be hospitalised.
Symptoms at the point of diagnosis were reported for 31 cases. Two cases were asymptomatic and remained so until tested negative. The asymptomatic cases were tested as part of screening following repatriation and during contact tracing respectively. Of the remaining 29, 20 reported fever, 14 reported cough and eight reported weakness. Additional symptoms reported included headaches (6 cases), sore throat (2), rhinorrhoea (2), shortness of breath (2), myalgia (1), diarrhoea (1) and nausea (1). Fever was reported as the sole symptom for nine cases. In 16 of 29 symptomatic cases, the symptoms at diagnosis were consistent with the case definition for acute respiratory infection [16] , although it is possible that cases presented additional symptoms after diagnosis and these were not reported.
Data on pre-existing conditions were reported for seven cases; five had no pre-existing conditions while one was reported to be obese and one had pre-existing cardiac disease. No data on clinical signs e.g. dyspnea etc. were reported for any of the 38 cases.
All hospitalised cases had a benign clinical evolution except four, two reported in Italy and two reported in France, all of whom developed viral pneumonia. All three cases who were aged 65 years or over were admitted to intensive care and required respiratory support and one French case died. The case who died was hospitalised for 21 days and required intensive care and mechanical ventilation for 19 days. The duration of hospitalisation was reported for 16 cases with a median of 13 days (range: 8-23 days). As at 21 February 2020, four cases were still hospitalised.
All cases were confirmed according to specific assays targeting at least two separate genes (envelope (E) gene as a screening test and RNA-dependent RNA polymerase (RdRp) gene or nucleoprotein (N) gene for confirmation) [8, 17] . The specimen types tested were reported for 27 cases: 15 had positive nasopharyngeal swabs, nine had positive throat swabs, three cases had positive sputum, two had a positive nasal swab, one case had a positive nasopharyngeal aspirate and one a positive endotracheal aspirate.
As at 09:00 on 21 February, few COVID-19 cases had been detected in Europe compared with Asia. However the situation is rapidly developing, with a large outbreak recently identified in northern Italy, with transmission in several municipalities and at least two deaths [18] . As at 5 March 2020, there are 4,250 cases including 113 deaths reported among 38 countries in the WHO European region [19] .
In our analysis of early cases, we observed transmission in two broad contexts: sporadic cases among travellers from China (14 cases) and cases who acquired infection due to subsequent local transmission in Europe (21 cases). Our analysis shows that the time from symptom onset to hospitalisation/case isolation was about 3 days longer for locally acquired cases than for imported cases. People returning from affected areas are likely to have a low threshold to seek care and be tested when symptomatic, however delays in identifying the index cases of the two clusters in France and Germany meant that locally acquired cases took longer to be detected and isolated. Once the exposure is determined and contacts identified and quarantined (171 contacts in France and 200 in Germany for the clusters in Haute-Savoie and Bavaria, respectively), further cases are likely to be rapidly detected and isolated when they develop symptoms [15, 20] . In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six were hospitalised after a mean of 2 days. Locally acquired cases require significant resources for contact tracing and quarantine, and countries should be prepared to allocate considerable public health resources during the containment phase, should local clusters emerge in their population. In addition, prompt sharing of information on cases and contacts through international notification systems such as the International Health Regulations (IHR) mechanism and the European Commission's European Early Warning and Response System is essential to contain international spread of infection.
All of the imported cases had a history of travel to China. This was consistent with the epidemiological situation in Asia, and supported the recommendation for testing of suspected cases with travel history to China and potentially other areas of presumed ongoing community transmission. The situation has evolved rapidly since then, however, and the number of countries reporting COVID-19 transmission increased rapidly, notably with a large outbreak in northern Italy with 3,089 cases reported as at 5 March [18, 19] . Testing of suspected cases based on geographical risk of importation needs to be complemented with additional approaches to ensure early detection of local circulation of COVID-19, including through testing of severe acute respiratory infections in hospitals irrespectively of travel history as recommended in the WHO case definition updated on 27 February 2020 [21] .
The clinical presentation observed in the cases in Europe is that of an acute respiratory infection. However, of the 31 cases with information on symptoms, 20 cases presented with fever and nine cases presented only with fever and no other symptoms. These findings, which are consistent with other published case series, have prompted ECDC to include fever among several clinical signs or symptoms indicative for the suspected case definition.
Three cases were aged 65 years or over. All required admission to intensive care and were tourists (imported cases). These findings could reflect the average older age of the tourist population compared with the local contacts exposed to infection in Europe and do not allow us to draw any conclusion on the proportion of severe cases that we could expect in the general population of Europe. Despite this, the finding of older individuals being at higher risk of a severe clinical course is consistent with the evidence from Chinese case series published so far although the majority of infections in China have been mild [22, 23] .
This preliminary analysis is based on the first reported cases of COVID-19 cases in the WHO European Region. Given the small sample size, and limited completeness for some variables, all the results presented should be interpreted with caution.
With increasing numbers of cases in Europe, data from surveillance and investigations in the region can build on the evidence from countries in Asia experiencing more widespread transmission particularly on disease spectrum and the proportion of infections with severe outcome [22] . Understanding the infection-severity is critical to help plan for the impact on the healthcare system and the wider population. Serological studies are vital to understand the proportion of cases who are asymptomatic. Hospital-based surveillance could help estimate the incidence of severe cases and identify risk factors for severity and death. Established hospital surveillance systems that are in place for influenza and other diseases in Europe may be expanded for this purpose. In addition, a number of countries in Europe are adapting and, in some cases, already using existing sentinel primary care based surveillance systems for influenza to detect community transmission of SARS-CoV-2. This approach will be used globally to help identify evidence of widespread community transmission and, should the virus spread and containment no longer be deemed feasible, to monitor intensity of disease transmission, trends and its geographical spread.
Additional research is needed to complement surveillance data to build knowledge on the infectious period, modes of transmission, basic and effective reproduction numbers, and effectiveness of prevention and case management options also in settings outside of China. Such special studies are being conducted globally, including a cohort study on citizens repatriated from China to Europe, with the aim to extrapolate disease incidence and risk factors for infection in areas with community transmission. Countries together with ECDC and WHO, should use all opportunities to address these questions in a coordinated fashion at the European and global level.
provided input to the outline, multiple versions of the manuscript and gave approval to the final draft. | Why are serological tests vital? | {
"answer_start": [
13911
],
"text": [
"to understand the proportion of cases who are asymptomatic."
]
} | false |
3,843 | First cases of coronavirus disease 2019 (COVID-19) in the WHO European Region, 24 January to 21 February 2020
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7068164/
SHA: ce358c18aac69fc83c7b2e9a7dca4a43b0f60e2e
Authors: Spiteri, Gianfranco; Fielding, James; Diercke, Michaela; Campese, Christine; Enouf, Vincent; Gaymard, Alexandre; Bella, Antonino; Sognamiglio, Paola; Sierra Moros, Maria José; Riutort, Antonio Nicolau; Demina, Yulia V.; Mahieu, Romain; Broas, Markku; Bengnér, Malin; Buda, Silke; Schilling, Julia; Filleul, Laurent; Lepoutre, Agnès; Saura, Christine; Mailles, Alexandra; Levy-Bruhl, Daniel; Coignard, Bruno; Bernard-Stoecklin, Sibylle; Behillil, Sylvie; van der Werf, Sylvie; Valette, Martine; Lina, Bruno; Riccardo, Flavia; Nicastri, Emanuele; Casas, Inmaculada; Larrauri, Amparo; Salom Castell, Magdalena; Pozo, Francisco; Maksyutov, Rinat A.; Martin, Charlotte; Van Ranst, Marc; Bossuyt, Nathalie; Siira, Lotta; Sane, Jussi; Tegmark-Wisell, Karin; Palmérus, Maria; Broberg, Eeva K.; Beauté, Julien; Jorgensen, Pernille; Bundle, Nick; Pereyaslov, Dmitriy; Adlhoch, Cornelia; Pukkila, Jukka; Pebody, Richard; Olsen, Sonja; Ciancio, Bruno Christian
Date: 2020-03-05
DOI: 10.2807/1560-7917.es.2020.25.9.2000178
License: cc-by
Abstract: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters’ index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
Text: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters' index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
A cluster of pneumonia of unknown origin was identified in Wuhan, China, in December 2019 [1] . On 12 January 2020, Chinese authorities shared the sequence of a novel coronavirus termed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) isolated from some clustered cases [2] . Since then, the disease caused by SARS-CoV-2 has been named coronavirus disease 2019 (COVID -19) . As at 21 February 2020, the virus had spread rapidly mostly within China but also to 28 other countries, including in the World Health Organization (WHO) European Region [3] [4] [5] .
Here we describe the epidemiology of the first cases of COVID-19 in this region, excluding cases reported in the United Kingdom (UK), as at 21 February 2020. The study includes a comparison between cases detected among travellers from China and cases whose infection was acquired due to subsequent local transmission.
On 27 January 2020, the European Centre for Disease Prevention and Control (ECDC) and the WHO Regional Office for Europe asked countries to complete a WHO standard COVID-19 case report form for all confirmed and probable cases according to WHO criteria [6] [7] [8] . The overall aim of surveillance at this time was to support the global strategy of containment of COVID-19 with rapid identification and follow-up of cases linked to affected countries in order to minimise onward transmission. The surveillance objectives were to: describe the key epidemiological and clinical characteristics of COVID-19 cases detected in Europe; inform country preparedness; and improve further case detection and management. Data collected included demographics, history of recent travel to affected areas, close contact with a probable or confirmed COVID-19 case, underlying conditions, signs and symptoms of disease at onset, type of specimens from which the virus was detected, and clinical outcome. The WHO case definition was adopted for surveillance: a confirmed case was a person with laboratory confirmation of SARS-CoV-2 infection (ECDC recommended two separate SARS-CoV-2 RT-PCR tests), irrespective of clinical signs and symptoms, whereas a probable case was a suspect case for whom testing for SARS-CoV-2 was inconclusive or positive using a pan-coronavirus assay [8] . By 31 January 2020, 47 laboratories in 31 countries, including 38 laboratories in 24 European Union and European Economic Area (EU/EEA) countries, had diagnostic capability for SARS-CoV-2 available (close to 60% of countries in the WHO European Region), with cross-border shipment arrangements in place for many of those lacking domestic testing capacity. The remaining six EU/EEA countries were expected to have diagnostic testing available by mid-February [9] .
As at 09:00 on 21 February 2020, 47 confirmed cases of COVID-19 were reported in the WHO European Region and one of these cases had died [4] . Data on 38 of these cases (i.e. all except the nine reported in the UK) are included in this analysis.
The first three cases detected were reported in France on 24 January 2020 and had onset of symptoms on 17, 19 and 23 January respectively [10] . The first death was reported on 15 February in France. As at 21 February, nine countries had reported cases ( Figure) : Belgium (1), Finland (1), France (12), Germany (16), Italy (3), Russia (2), Spain (2), Sweden (1) and the UK (9 -not included further).
The place of infection (assessed at national level based on an incubation period presumed to be up to 14 days [11] , travel history and contact with probable or confirmed cases as per the case definition) was reported for 35 cases (missing for three cases), of whom 14 were infected in China (Hubei province: 10 cases; Shandong province: one case; province not reported for three cases). The remaining 21 cases were infected in Europe. Of these, 14 were linked to a cluster in Bavaria, Germany, and seven to a cluster in Haute-Savoie, France [12, 13] . Cases from the Bavarian cluster were reported from Germany and Spain, whereas cases from the Haute-Savoie cluster were reported from France All but two cases were hospitalised (35 of 37 where information on hospitalisation was reported), although it is likely that most were hospitalised to isolate the person rather than because of severe disease. The time from onset of symptoms to hospitalisation (and isolation) ranged between 0 and 10 days with a mean of 3.7 days (reported for 29 cases). The mean number of days to hospitalisation was 2.5 days for cases imported from China, but 4.6 days for those infected in Europe. This was mostly a result of delays in identifying the index cases of the two clusters in France and Germany. In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six took only a mean of 2 days to be hospitalised.
Symptoms at the point of diagnosis were reported for 31 cases. Two cases were asymptomatic and remained so until tested negative. The asymptomatic cases were tested as part of screening following repatriation and during contact tracing respectively. Of the remaining 29, 20 reported fever, 14 reported cough and eight reported weakness. Additional symptoms reported included headaches (6 cases), sore throat (2), rhinorrhoea (2), shortness of breath (2), myalgia (1), diarrhoea (1) and nausea (1). Fever was reported as the sole symptom for nine cases. In 16 of 29 symptomatic cases, the symptoms at diagnosis were consistent with the case definition for acute respiratory infection [16] , although it is possible that cases presented additional symptoms after diagnosis and these were not reported.
Data on pre-existing conditions were reported for seven cases; five had no pre-existing conditions while one was reported to be obese and one had pre-existing cardiac disease. No data on clinical signs e.g. dyspnea etc. were reported for any of the 38 cases.
All hospitalised cases had a benign clinical evolution except four, two reported in Italy and two reported in France, all of whom developed viral pneumonia. All three cases who were aged 65 years or over were admitted to intensive care and required respiratory support and one French case died. The case who died was hospitalised for 21 days and required intensive care and mechanical ventilation for 19 days. The duration of hospitalisation was reported for 16 cases with a median of 13 days (range: 8-23 days). As at 21 February 2020, four cases were still hospitalised.
All cases were confirmed according to specific assays targeting at least two separate genes (envelope (E) gene as a screening test and RNA-dependent RNA polymerase (RdRp) gene or nucleoprotein (N) gene for confirmation) [8, 17] . The specimen types tested were reported for 27 cases: 15 had positive nasopharyngeal swabs, nine had positive throat swabs, three cases had positive sputum, two had a positive nasal swab, one case had a positive nasopharyngeal aspirate and one a positive endotracheal aspirate.
As at 09:00 on 21 February, few COVID-19 cases had been detected in Europe compared with Asia. However the situation is rapidly developing, with a large outbreak recently identified in northern Italy, with transmission in several municipalities and at least two deaths [18] . As at 5 March 2020, there are 4,250 cases including 113 deaths reported among 38 countries in the WHO European region [19] .
In our analysis of early cases, we observed transmission in two broad contexts: sporadic cases among travellers from China (14 cases) and cases who acquired infection due to subsequent local transmission in Europe (21 cases). Our analysis shows that the time from symptom onset to hospitalisation/case isolation was about 3 days longer for locally acquired cases than for imported cases. People returning from affected areas are likely to have a low threshold to seek care and be tested when symptomatic, however delays in identifying the index cases of the two clusters in France and Germany meant that locally acquired cases took longer to be detected and isolated. Once the exposure is determined and contacts identified and quarantined (171 contacts in France and 200 in Germany for the clusters in Haute-Savoie and Bavaria, respectively), further cases are likely to be rapidly detected and isolated when they develop symptoms [15, 20] . In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six were hospitalised after a mean of 2 days. Locally acquired cases require significant resources for contact tracing and quarantine, and countries should be prepared to allocate considerable public health resources during the containment phase, should local clusters emerge in their population. In addition, prompt sharing of information on cases and contacts through international notification systems such as the International Health Regulations (IHR) mechanism and the European Commission's European Early Warning and Response System is essential to contain international spread of infection.
All of the imported cases had a history of travel to China. This was consistent with the epidemiological situation in Asia, and supported the recommendation for testing of suspected cases with travel history to China and potentially other areas of presumed ongoing community transmission. The situation has evolved rapidly since then, however, and the number of countries reporting COVID-19 transmission increased rapidly, notably with a large outbreak in northern Italy with 3,089 cases reported as at 5 March [18, 19] . Testing of suspected cases based on geographical risk of importation needs to be complemented with additional approaches to ensure early detection of local circulation of COVID-19, including through testing of severe acute respiratory infections in hospitals irrespectively of travel history as recommended in the WHO case definition updated on 27 February 2020 [21] .
The clinical presentation observed in the cases in Europe is that of an acute respiratory infection. However, of the 31 cases with information on symptoms, 20 cases presented with fever and nine cases presented only with fever and no other symptoms. These findings, which are consistent with other published case series, have prompted ECDC to include fever among several clinical signs or symptoms indicative for the suspected case definition.
Three cases were aged 65 years or over. All required admission to intensive care and were tourists (imported cases). These findings could reflect the average older age of the tourist population compared with the local contacts exposed to infection in Europe and do not allow us to draw any conclusion on the proportion of severe cases that we could expect in the general population of Europe. Despite this, the finding of older individuals being at higher risk of a severe clinical course is consistent with the evidence from Chinese case series published so far although the majority of infections in China have been mild [22, 23] .
This preliminary analysis is based on the first reported cases of COVID-19 cases in the WHO European Region. Given the small sample size, and limited completeness for some variables, all the results presented should be interpreted with caution.
With increasing numbers of cases in Europe, data from surveillance and investigations in the region can build on the evidence from countries in Asia experiencing more widespread transmission particularly on disease spectrum and the proportion of infections with severe outcome [22] . Understanding the infection-severity is critical to help plan for the impact on the healthcare system and the wider population. Serological studies are vital to understand the proportion of cases who are asymptomatic. Hospital-based surveillance could help estimate the incidence of severe cases and identify risk factors for severity and death. Established hospital surveillance systems that are in place for influenza and other diseases in Europe may be expanded for this purpose. In addition, a number of countries in Europe are adapting and, in some cases, already using existing sentinel primary care based surveillance systems for influenza to detect community transmission of SARS-CoV-2. This approach will be used globally to help identify evidence of widespread community transmission and, should the virus spread and containment no longer be deemed feasible, to monitor intensity of disease transmission, trends and its geographical spread.
Additional research is needed to complement surveillance data to build knowledge on the infectious period, modes of transmission, basic and effective reproduction numbers, and effectiveness of prevention and case management options also in settings outside of China. Such special studies are being conducted globally, including a cohort study on citizens repatriated from China to Europe, with the aim to extrapolate disease incidence and risk factors for infection in areas with community transmission. Countries together with ECDC and WHO, should use all opportunities to address these questions in a coordinated fashion at the European and global level.
provided input to the outline, multiple versions of the manuscript and gave approval to the final draft. | How can hospital based surveillance help? | {
"answer_start": [
14005
],
"text": [
"help estimate the incidence of severe cases and identify risk factors for severity and death"
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3,844 | First cases of coronavirus disease 2019 (COVID-19) in the WHO European Region, 24 January to 21 February 2020
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7068164/
SHA: ce358c18aac69fc83c7b2e9a7dca4a43b0f60e2e
Authors: Spiteri, Gianfranco; Fielding, James; Diercke, Michaela; Campese, Christine; Enouf, Vincent; Gaymard, Alexandre; Bella, Antonino; Sognamiglio, Paola; Sierra Moros, Maria José; Riutort, Antonio Nicolau; Demina, Yulia V.; Mahieu, Romain; Broas, Markku; Bengnér, Malin; Buda, Silke; Schilling, Julia; Filleul, Laurent; Lepoutre, Agnès; Saura, Christine; Mailles, Alexandra; Levy-Bruhl, Daniel; Coignard, Bruno; Bernard-Stoecklin, Sibylle; Behillil, Sylvie; van der Werf, Sylvie; Valette, Martine; Lina, Bruno; Riccardo, Flavia; Nicastri, Emanuele; Casas, Inmaculada; Larrauri, Amparo; Salom Castell, Magdalena; Pozo, Francisco; Maksyutov, Rinat A.; Martin, Charlotte; Van Ranst, Marc; Bossuyt, Nathalie; Siira, Lotta; Sane, Jussi; Tegmark-Wisell, Karin; Palmérus, Maria; Broberg, Eeva K.; Beauté, Julien; Jorgensen, Pernille; Bundle, Nick; Pereyaslov, Dmitriy; Adlhoch, Cornelia; Pukkila, Jukka; Pebody, Richard; Olsen, Sonja; Ciancio, Bruno Christian
Date: 2020-03-05
DOI: 10.2807/1560-7917.es.2020.25.9.2000178
License: cc-by
Abstract: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters’ index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
Text: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters' index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
A cluster of pneumonia of unknown origin was identified in Wuhan, China, in December 2019 [1] . On 12 January 2020, Chinese authorities shared the sequence of a novel coronavirus termed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) isolated from some clustered cases [2] . Since then, the disease caused by SARS-CoV-2 has been named coronavirus disease 2019 (COVID -19) . As at 21 February 2020, the virus had spread rapidly mostly within China but also to 28 other countries, including in the World Health Organization (WHO) European Region [3] [4] [5] .
Here we describe the epidemiology of the first cases of COVID-19 in this region, excluding cases reported in the United Kingdom (UK), as at 21 February 2020. The study includes a comparison between cases detected among travellers from China and cases whose infection was acquired due to subsequent local transmission.
On 27 January 2020, the European Centre for Disease Prevention and Control (ECDC) and the WHO Regional Office for Europe asked countries to complete a WHO standard COVID-19 case report form for all confirmed and probable cases according to WHO criteria [6] [7] [8] . The overall aim of surveillance at this time was to support the global strategy of containment of COVID-19 with rapid identification and follow-up of cases linked to affected countries in order to minimise onward transmission. The surveillance objectives were to: describe the key epidemiological and clinical characteristics of COVID-19 cases detected in Europe; inform country preparedness; and improve further case detection and management. Data collected included demographics, history of recent travel to affected areas, close contact with a probable or confirmed COVID-19 case, underlying conditions, signs and symptoms of disease at onset, type of specimens from which the virus was detected, and clinical outcome. The WHO case definition was adopted for surveillance: a confirmed case was a person with laboratory confirmation of SARS-CoV-2 infection (ECDC recommended two separate SARS-CoV-2 RT-PCR tests), irrespective of clinical signs and symptoms, whereas a probable case was a suspect case for whom testing for SARS-CoV-2 was inconclusive or positive using a pan-coronavirus assay [8] . By 31 January 2020, 47 laboratories in 31 countries, including 38 laboratories in 24 European Union and European Economic Area (EU/EEA) countries, had diagnostic capability for SARS-CoV-2 available (close to 60% of countries in the WHO European Region), with cross-border shipment arrangements in place for many of those lacking domestic testing capacity. The remaining six EU/EEA countries were expected to have diagnostic testing available by mid-February [9] .
As at 09:00 on 21 February 2020, 47 confirmed cases of COVID-19 were reported in the WHO European Region and one of these cases had died [4] . Data on 38 of these cases (i.e. all except the nine reported in the UK) are included in this analysis.
The first three cases detected were reported in France on 24 January 2020 and had onset of symptoms on 17, 19 and 23 January respectively [10] . The first death was reported on 15 February in France. As at 21 February, nine countries had reported cases ( Figure) : Belgium (1), Finland (1), France (12), Germany (16), Italy (3), Russia (2), Spain (2), Sweden (1) and the UK (9 -not included further).
The place of infection (assessed at national level based on an incubation period presumed to be up to 14 days [11] , travel history and contact with probable or confirmed cases as per the case definition) was reported for 35 cases (missing for three cases), of whom 14 were infected in China (Hubei province: 10 cases; Shandong province: one case; province not reported for three cases). The remaining 21 cases were infected in Europe. Of these, 14 were linked to a cluster in Bavaria, Germany, and seven to a cluster in Haute-Savoie, France [12, 13] . Cases from the Bavarian cluster were reported from Germany and Spain, whereas cases from the Haute-Savoie cluster were reported from France All but two cases were hospitalised (35 of 37 where information on hospitalisation was reported), although it is likely that most were hospitalised to isolate the person rather than because of severe disease. The time from onset of symptoms to hospitalisation (and isolation) ranged between 0 and 10 days with a mean of 3.7 days (reported for 29 cases). The mean number of days to hospitalisation was 2.5 days for cases imported from China, but 4.6 days for those infected in Europe. This was mostly a result of delays in identifying the index cases of the two clusters in France and Germany. In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six took only a mean of 2 days to be hospitalised.
Symptoms at the point of diagnosis were reported for 31 cases. Two cases were asymptomatic and remained so until tested negative. The asymptomatic cases were tested as part of screening following repatriation and during contact tracing respectively. Of the remaining 29, 20 reported fever, 14 reported cough and eight reported weakness. Additional symptoms reported included headaches (6 cases), sore throat (2), rhinorrhoea (2), shortness of breath (2), myalgia (1), diarrhoea (1) and nausea (1). Fever was reported as the sole symptom for nine cases. In 16 of 29 symptomatic cases, the symptoms at diagnosis were consistent with the case definition for acute respiratory infection [16] , although it is possible that cases presented additional symptoms after diagnosis and these were not reported.
Data on pre-existing conditions were reported for seven cases; five had no pre-existing conditions while one was reported to be obese and one had pre-existing cardiac disease. No data on clinical signs e.g. dyspnea etc. were reported for any of the 38 cases.
All hospitalised cases had a benign clinical evolution except four, two reported in Italy and two reported in France, all of whom developed viral pneumonia. All three cases who were aged 65 years or over were admitted to intensive care and required respiratory support and one French case died. The case who died was hospitalised for 21 days and required intensive care and mechanical ventilation for 19 days. The duration of hospitalisation was reported for 16 cases with a median of 13 days (range: 8-23 days). As at 21 February 2020, four cases were still hospitalised.
All cases were confirmed according to specific assays targeting at least two separate genes (envelope (E) gene as a screening test and RNA-dependent RNA polymerase (RdRp) gene or nucleoprotein (N) gene for confirmation) [8, 17] . The specimen types tested were reported for 27 cases: 15 had positive nasopharyngeal swabs, nine had positive throat swabs, three cases had positive sputum, two had a positive nasal swab, one case had a positive nasopharyngeal aspirate and one a positive endotracheal aspirate.
As at 09:00 on 21 February, few COVID-19 cases had been detected in Europe compared with Asia. However the situation is rapidly developing, with a large outbreak recently identified in northern Italy, with transmission in several municipalities and at least two deaths [18] . As at 5 March 2020, there are 4,250 cases including 113 deaths reported among 38 countries in the WHO European region [19] .
In our analysis of early cases, we observed transmission in two broad contexts: sporadic cases among travellers from China (14 cases) and cases who acquired infection due to subsequent local transmission in Europe (21 cases). Our analysis shows that the time from symptom onset to hospitalisation/case isolation was about 3 days longer for locally acquired cases than for imported cases. People returning from affected areas are likely to have a low threshold to seek care and be tested when symptomatic, however delays in identifying the index cases of the two clusters in France and Germany meant that locally acquired cases took longer to be detected and isolated. Once the exposure is determined and contacts identified and quarantined (171 contacts in France and 200 in Germany for the clusters in Haute-Savoie and Bavaria, respectively), further cases are likely to be rapidly detected and isolated when they develop symptoms [15, 20] . In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six were hospitalised after a mean of 2 days. Locally acquired cases require significant resources for contact tracing and quarantine, and countries should be prepared to allocate considerable public health resources during the containment phase, should local clusters emerge in their population. In addition, prompt sharing of information on cases and contacts through international notification systems such as the International Health Regulations (IHR) mechanism and the European Commission's European Early Warning and Response System is essential to contain international spread of infection.
All of the imported cases had a history of travel to China. This was consistent with the epidemiological situation in Asia, and supported the recommendation for testing of suspected cases with travel history to China and potentially other areas of presumed ongoing community transmission. The situation has evolved rapidly since then, however, and the number of countries reporting COVID-19 transmission increased rapidly, notably with a large outbreak in northern Italy with 3,089 cases reported as at 5 March [18, 19] . Testing of suspected cases based on geographical risk of importation needs to be complemented with additional approaches to ensure early detection of local circulation of COVID-19, including through testing of severe acute respiratory infections in hospitals irrespectively of travel history as recommended in the WHO case definition updated on 27 February 2020 [21] .
The clinical presentation observed in the cases in Europe is that of an acute respiratory infection. However, of the 31 cases with information on symptoms, 20 cases presented with fever and nine cases presented only with fever and no other symptoms. These findings, which are consistent with other published case series, have prompted ECDC to include fever among several clinical signs or symptoms indicative for the suspected case definition.
Three cases were aged 65 years or over. All required admission to intensive care and were tourists (imported cases). These findings could reflect the average older age of the tourist population compared with the local contacts exposed to infection in Europe and do not allow us to draw any conclusion on the proportion of severe cases that we could expect in the general population of Europe. Despite this, the finding of older individuals being at higher risk of a severe clinical course is consistent with the evidence from Chinese case series published so far although the majority of infections in China have been mild [22, 23] .
This preliminary analysis is based on the first reported cases of COVID-19 cases in the WHO European Region. Given the small sample size, and limited completeness for some variables, all the results presented should be interpreted with caution.
With increasing numbers of cases in Europe, data from surveillance and investigations in the region can build on the evidence from countries in Asia experiencing more widespread transmission particularly on disease spectrum and the proportion of infections with severe outcome [22] . Understanding the infection-severity is critical to help plan for the impact on the healthcare system and the wider population. Serological studies are vital to understand the proportion of cases who are asymptomatic. Hospital-based surveillance could help estimate the incidence of severe cases and identify risk factors for severity and death. Established hospital surveillance systems that are in place for influenza and other diseases in Europe may be expanded for this purpose. In addition, a number of countries in Europe are adapting and, in some cases, already using existing sentinel primary care based surveillance systems for influenza to detect community transmission of SARS-CoV-2. This approach will be used globally to help identify evidence of widespread community transmission and, should the virus spread and containment no longer be deemed feasible, to monitor intensity of disease transmission, trends and its geographical spread.
Additional research is needed to complement surveillance data to build knowledge on the infectious period, modes of transmission, basic and effective reproduction numbers, and effectiveness of prevention and case management options also in settings outside of China. Such special studies are being conducted globally, including a cohort study on citizens repatriated from China to Europe, with the aim to extrapolate disease incidence and risk factors for infection in areas with community transmission. Countries together with ECDC and WHO, should use all opportunities to address these questions in a coordinated fashion at the European and global level.
provided input to the outline, multiple versions of the manuscript and gave approval to the final draft. | How can present systems of surveillance be used? | {
"answer_start": [
14099
],
"text": [
"Established hospital surveillance systems that are in place for influenza and other diseases in Europe may be expanded for this purpose. In addition, a number of countries in Europe are adapting and, in some cases, already using existing sentinel primary care based surveillance systems for influenza to detect community transmission of SARS-CoV-2"
]
} | false |
3,845 | First cases of coronavirus disease 2019 (COVID-19) in the WHO European Region, 24 January to 21 February 2020
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7068164/
SHA: ce358c18aac69fc83c7b2e9a7dca4a43b0f60e2e
Authors: Spiteri, Gianfranco; Fielding, James; Diercke, Michaela; Campese, Christine; Enouf, Vincent; Gaymard, Alexandre; Bella, Antonino; Sognamiglio, Paola; Sierra Moros, Maria José; Riutort, Antonio Nicolau; Demina, Yulia V.; Mahieu, Romain; Broas, Markku; Bengnér, Malin; Buda, Silke; Schilling, Julia; Filleul, Laurent; Lepoutre, Agnès; Saura, Christine; Mailles, Alexandra; Levy-Bruhl, Daniel; Coignard, Bruno; Bernard-Stoecklin, Sibylle; Behillil, Sylvie; van der Werf, Sylvie; Valette, Martine; Lina, Bruno; Riccardo, Flavia; Nicastri, Emanuele; Casas, Inmaculada; Larrauri, Amparo; Salom Castell, Magdalena; Pozo, Francisco; Maksyutov, Rinat A.; Martin, Charlotte; Van Ranst, Marc; Bossuyt, Nathalie; Siira, Lotta; Sane, Jussi; Tegmark-Wisell, Karin; Palmérus, Maria; Broberg, Eeva K.; Beauté, Julien; Jorgensen, Pernille; Bundle, Nick; Pereyaslov, Dmitriy; Adlhoch, Cornelia; Pukkila, Jukka; Pebody, Richard; Olsen, Sonja; Ciancio, Bruno Christian
Date: 2020-03-05
DOI: 10.2807/1560-7917.es.2020.25.9.2000178
License: cc-by
Abstract: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters’ index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
Text: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters' index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
A cluster of pneumonia of unknown origin was identified in Wuhan, China, in December 2019 [1] . On 12 January 2020, Chinese authorities shared the sequence of a novel coronavirus termed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) isolated from some clustered cases [2] . Since then, the disease caused by SARS-CoV-2 has been named coronavirus disease 2019 (COVID -19) . As at 21 February 2020, the virus had spread rapidly mostly within China but also to 28 other countries, including in the World Health Organization (WHO) European Region [3] [4] [5] .
Here we describe the epidemiology of the first cases of COVID-19 in this region, excluding cases reported in the United Kingdom (UK), as at 21 February 2020. The study includes a comparison between cases detected among travellers from China and cases whose infection was acquired due to subsequent local transmission.
On 27 January 2020, the European Centre for Disease Prevention and Control (ECDC) and the WHO Regional Office for Europe asked countries to complete a WHO standard COVID-19 case report form for all confirmed and probable cases according to WHO criteria [6] [7] [8] . The overall aim of surveillance at this time was to support the global strategy of containment of COVID-19 with rapid identification and follow-up of cases linked to affected countries in order to minimise onward transmission. The surveillance objectives were to: describe the key epidemiological and clinical characteristics of COVID-19 cases detected in Europe; inform country preparedness; and improve further case detection and management. Data collected included demographics, history of recent travel to affected areas, close contact with a probable or confirmed COVID-19 case, underlying conditions, signs and symptoms of disease at onset, type of specimens from which the virus was detected, and clinical outcome. The WHO case definition was adopted for surveillance: a confirmed case was a person with laboratory confirmation of SARS-CoV-2 infection (ECDC recommended two separate SARS-CoV-2 RT-PCR tests), irrespective of clinical signs and symptoms, whereas a probable case was a suspect case for whom testing for SARS-CoV-2 was inconclusive or positive using a pan-coronavirus assay [8] . By 31 January 2020, 47 laboratories in 31 countries, including 38 laboratories in 24 European Union and European Economic Area (EU/EEA) countries, had diagnostic capability for SARS-CoV-2 available (close to 60% of countries in the WHO European Region), with cross-border shipment arrangements in place for many of those lacking domestic testing capacity. The remaining six EU/EEA countries were expected to have diagnostic testing available by mid-February [9] .
As at 09:00 on 21 February 2020, 47 confirmed cases of COVID-19 were reported in the WHO European Region and one of these cases had died [4] . Data on 38 of these cases (i.e. all except the nine reported in the UK) are included in this analysis.
The first three cases detected were reported in France on 24 January 2020 and had onset of symptoms on 17, 19 and 23 January respectively [10] . The first death was reported on 15 February in France. As at 21 February, nine countries had reported cases ( Figure) : Belgium (1), Finland (1), France (12), Germany (16), Italy (3), Russia (2), Spain (2), Sweden (1) and the UK (9 -not included further).
The place of infection (assessed at national level based on an incubation period presumed to be up to 14 days [11] , travel history and contact with probable or confirmed cases as per the case definition) was reported for 35 cases (missing for three cases), of whom 14 were infected in China (Hubei province: 10 cases; Shandong province: one case; province not reported for three cases). The remaining 21 cases were infected in Europe. Of these, 14 were linked to a cluster in Bavaria, Germany, and seven to a cluster in Haute-Savoie, France [12, 13] . Cases from the Bavarian cluster were reported from Germany and Spain, whereas cases from the Haute-Savoie cluster were reported from France All but two cases were hospitalised (35 of 37 where information on hospitalisation was reported), although it is likely that most were hospitalised to isolate the person rather than because of severe disease. The time from onset of symptoms to hospitalisation (and isolation) ranged between 0 and 10 days with a mean of 3.7 days (reported for 29 cases). The mean number of days to hospitalisation was 2.5 days for cases imported from China, but 4.6 days for those infected in Europe. This was mostly a result of delays in identifying the index cases of the two clusters in France and Germany. In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six took only a mean of 2 days to be hospitalised.
Symptoms at the point of diagnosis were reported for 31 cases. Two cases were asymptomatic and remained so until tested negative. The asymptomatic cases were tested as part of screening following repatriation and during contact tracing respectively. Of the remaining 29, 20 reported fever, 14 reported cough and eight reported weakness. Additional symptoms reported included headaches (6 cases), sore throat (2), rhinorrhoea (2), shortness of breath (2), myalgia (1), diarrhoea (1) and nausea (1). Fever was reported as the sole symptom for nine cases. In 16 of 29 symptomatic cases, the symptoms at diagnosis were consistent with the case definition for acute respiratory infection [16] , although it is possible that cases presented additional symptoms after diagnosis and these were not reported.
Data on pre-existing conditions were reported for seven cases; five had no pre-existing conditions while one was reported to be obese and one had pre-existing cardiac disease. No data on clinical signs e.g. dyspnea etc. were reported for any of the 38 cases.
All hospitalised cases had a benign clinical evolution except four, two reported in Italy and two reported in France, all of whom developed viral pneumonia. All three cases who were aged 65 years or over were admitted to intensive care and required respiratory support and one French case died. The case who died was hospitalised for 21 days and required intensive care and mechanical ventilation for 19 days. The duration of hospitalisation was reported for 16 cases with a median of 13 days (range: 8-23 days). As at 21 February 2020, four cases were still hospitalised.
All cases were confirmed according to specific assays targeting at least two separate genes (envelope (E) gene as a screening test and RNA-dependent RNA polymerase (RdRp) gene or nucleoprotein (N) gene for confirmation) [8, 17] . The specimen types tested were reported for 27 cases: 15 had positive nasopharyngeal swabs, nine had positive throat swabs, three cases had positive sputum, two had a positive nasal swab, one case had a positive nasopharyngeal aspirate and one a positive endotracheal aspirate.
As at 09:00 on 21 February, few COVID-19 cases had been detected in Europe compared with Asia. However the situation is rapidly developing, with a large outbreak recently identified in northern Italy, with transmission in several municipalities and at least two deaths [18] . As at 5 March 2020, there are 4,250 cases including 113 deaths reported among 38 countries in the WHO European region [19] .
In our analysis of early cases, we observed transmission in two broad contexts: sporadic cases among travellers from China (14 cases) and cases who acquired infection due to subsequent local transmission in Europe (21 cases). Our analysis shows that the time from symptom onset to hospitalisation/case isolation was about 3 days longer for locally acquired cases than for imported cases. People returning from affected areas are likely to have a low threshold to seek care and be tested when symptomatic, however delays in identifying the index cases of the two clusters in France and Germany meant that locally acquired cases took longer to be detected and isolated. Once the exposure is determined and contacts identified and quarantined (171 contacts in France and 200 in Germany for the clusters in Haute-Savoie and Bavaria, respectively), further cases are likely to be rapidly detected and isolated when they develop symptoms [15, 20] . In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six were hospitalised after a mean of 2 days. Locally acquired cases require significant resources for contact tracing and quarantine, and countries should be prepared to allocate considerable public health resources during the containment phase, should local clusters emerge in their population. In addition, prompt sharing of information on cases and contacts through international notification systems such as the International Health Regulations (IHR) mechanism and the European Commission's European Early Warning and Response System is essential to contain international spread of infection.
All of the imported cases had a history of travel to China. This was consistent with the epidemiological situation in Asia, and supported the recommendation for testing of suspected cases with travel history to China and potentially other areas of presumed ongoing community transmission. The situation has evolved rapidly since then, however, and the number of countries reporting COVID-19 transmission increased rapidly, notably with a large outbreak in northern Italy with 3,089 cases reported as at 5 March [18, 19] . Testing of suspected cases based on geographical risk of importation needs to be complemented with additional approaches to ensure early detection of local circulation of COVID-19, including through testing of severe acute respiratory infections in hospitals irrespectively of travel history as recommended in the WHO case definition updated on 27 February 2020 [21] .
The clinical presentation observed in the cases in Europe is that of an acute respiratory infection. However, of the 31 cases with information on symptoms, 20 cases presented with fever and nine cases presented only with fever and no other symptoms. These findings, which are consistent with other published case series, have prompted ECDC to include fever among several clinical signs or symptoms indicative for the suspected case definition.
Three cases were aged 65 years or over. All required admission to intensive care and were tourists (imported cases). These findings could reflect the average older age of the tourist population compared with the local contacts exposed to infection in Europe and do not allow us to draw any conclusion on the proportion of severe cases that we could expect in the general population of Europe. Despite this, the finding of older individuals being at higher risk of a severe clinical course is consistent with the evidence from Chinese case series published so far although the majority of infections in China have been mild [22, 23] .
This preliminary analysis is based on the first reported cases of COVID-19 cases in the WHO European Region. Given the small sample size, and limited completeness for some variables, all the results presented should be interpreted with caution.
With increasing numbers of cases in Europe, data from surveillance and investigations in the region can build on the evidence from countries in Asia experiencing more widespread transmission particularly on disease spectrum and the proportion of infections with severe outcome [22] . Understanding the infection-severity is critical to help plan for the impact on the healthcare system and the wider population. Serological studies are vital to understand the proportion of cases who are asymptomatic. Hospital-based surveillance could help estimate the incidence of severe cases and identify risk factors for severity and death. Established hospital surveillance systems that are in place for influenza and other diseases in Europe may be expanded for this purpose. In addition, a number of countries in Europe are adapting and, in some cases, already using existing sentinel primary care based surveillance systems for influenza to detect community transmission of SARS-CoV-2. This approach will be used globally to help identify evidence of widespread community transmission and, should the virus spread and containment no longer be deemed feasible, to monitor intensity of disease transmission, trends and its geographical spread.
Additional research is needed to complement surveillance data to build knowledge on the infectious period, modes of transmission, basic and effective reproduction numbers, and effectiveness of prevention and case management options also in settings outside of China. Such special studies are being conducted globally, including a cohort study on citizens repatriated from China to Europe, with the aim to extrapolate disease incidence and risk factors for infection in areas with community transmission. Countries together with ECDC and WHO, should use all opportunities to address these questions in a coordinated fashion at the European and global level.
provided input to the outline, multiple versions of the manuscript and gave approval to the final draft. | How will this approach used? | {
"answer_start": [
14462
],
"text": [
"will be used globally to help identify evidence of widespread community transmission and, should the virus spread and containment no longer be deemed feasible, to monitor intensity of disease transmission, trends and its geographical spread."
]
} | false |
3,846 | First cases of coronavirus disease 2019 (COVID-19) in the WHO European Region, 24 January to 21 February 2020
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7068164/
SHA: ce358c18aac69fc83c7b2e9a7dca4a43b0f60e2e
Authors: Spiteri, Gianfranco; Fielding, James; Diercke, Michaela; Campese, Christine; Enouf, Vincent; Gaymard, Alexandre; Bella, Antonino; Sognamiglio, Paola; Sierra Moros, Maria José; Riutort, Antonio Nicolau; Demina, Yulia V.; Mahieu, Romain; Broas, Markku; Bengnér, Malin; Buda, Silke; Schilling, Julia; Filleul, Laurent; Lepoutre, Agnès; Saura, Christine; Mailles, Alexandra; Levy-Bruhl, Daniel; Coignard, Bruno; Bernard-Stoecklin, Sibylle; Behillil, Sylvie; van der Werf, Sylvie; Valette, Martine; Lina, Bruno; Riccardo, Flavia; Nicastri, Emanuele; Casas, Inmaculada; Larrauri, Amparo; Salom Castell, Magdalena; Pozo, Francisco; Maksyutov, Rinat A.; Martin, Charlotte; Van Ranst, Marc; Bossuyt, Nathalie; Siira, Lotta; Sane, Jussi; Tegmark-Wisell, Karin; Palmérus, Maria; Broberg, Eeva K.; Beauté, Julien; Jorgensen, Pernille; Bundle, Nick; Pereyaslov, Dmitriy; Adlhoch, Cornelia; Pukkila, Jukka; Pebody, Richard; Olsen, Sonja; Ciancio, Bruno Christian
Date: 2020-03-05
DOI: 10.2807/1560-7917.es.2020.25.9.2000178
License: cc-by
Abstract: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters’ index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
Text: In the WHO European Region, COVID-19 surveillance was implemented 27 January 2020. We detail the first European cases. As at 21 February, nine European countries reported 47 cases. Among 38 cases studied, 21 were linked to two clusters in Germany and France, 14 were infected in China. Median case age was 42 years; 25 were male. Late detection of the clusters' index cases delayed isolation of further local cases. As at 5 March, there were 4,250 cases.
A cluster of pneumonia of unknown origin was identified in Wuhan, China, in December 2019 [1] . On 12 January 2020, Chinese authorities shared the sequence of a novel coronavirus termed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) isolated from some clustered cases [2] . Since then, the disease caused by SARS-CoV-2 has been named coronavirus disease 2019 (COVID -19) . As at 21 February 2020, the virus had spread rapidly mostly within China but also to 28 other countries, including in the World Health Organization (WHO) European Region [3] [4] [5] .
Here we describe the epidemiology of the first cases of COVID-19 in this region, excluding cases reported in the United Kingdom (UK), as at 21 February 2020. The study includes a comparison between cases detected among travellers from China and cases whose infection was acquired due to subsequent local transmission.
On 27 January 2020, the European Centre for Disease Prevention and Control (ECDC) and the WHO Regional Office for Europe asked countries to complete a WHO standard COVID-19 case report form for all confirmed and probable cases according to WHO criteria [6] [7] [8] . The overall aim of surveillance at this time was to support the global strategy of containment of COVID-19 with rapid identification and follow-up of cases linked to affected countries in order to minimise onward transmission. The surveillance objectives were to: describe the key epidemiological and clinical characteristics of COVID-19 cases detected in Europe; inform country preparedness; and improve further case detection and management. Data collected included demographics, history of recent travel to affected areas, close contact with a probable or confirmed COVID-19 case, underlying conditions, signs and symptoms of disease at onset, type of specimens from which the virus was detected, and clinical outcome. The WHO case definition was adopted for surveillance: a confirmed case was a person with laboratory confirmation of SARS-CoV-2 infection (ECDC recommended two separate SARS-CoV-2 RT-PCR tests), irrespective of clinical signs and symptoms, whereas a probable case was a suspect case for whom testing for SARS-CoV-2 was inconclusive or positive using a pan-coronavirus assay [8] . By 31 January 2020, 47 laboratories in 31 countries, including 38 laboratories in 24 European Union and European Economic Area (EU/EEA) countries, had diagnostic capability for SARS-CoV-2 available (close to 60% of countries in the WHO European Region), with cross-border shipment arrangements in place for many of those lacking domestic testing capacity. The remaining six EU/EEA countries were expected to have diagnostic testing available by mid-February [9] .
As at 09:00 on 21 February 2020, 47 confirmed cases of COVID-19 were reported in the WHO European Region and one of these cases had died [4] . Data on 38 of these cases (i.e. all except the nine reported in the UK) are included in this analysis.
The first three cases detected were reported in France on 24 January 2020 and had onset of symptoms on 17, 19 and 23 January respectively [10] . The first death was reported on 15 February in France. As at 21 February, nine countries had reported cases ( Figure) : Belgium (1), Finland (1), France (12), Germany (16), Italy (3), Russia (2), Spain (2), Sweden (1) and the UK (9 -not included further).
The place of infection (assessed at national level based on an incubation period presumed to be up to 14 days [11] , travel history and contact with probable or confirmed cases as per the case definition) was reported for 35 cases (missing for three cases), of whom 14 were infected in China (Hubei province: 10 cases; Shandong province: one case; province not reported for three cases). The remaining 21 cases were infected in Europe. Of these, 14 were linked to a cluster in Bavaria, Germany, and seven to a cluster in Haute-Savoie, France [12, 13] . Cases from the Bavarian cluster were reported from Germany and Spain, whereas cases from the Haute-Savoie cluster were reported from France All but two cases were hospitalised (35 of 37 where information on hospitalisation was reported), although it is likely that most were hospitalised to isolate the person rather than because of severe disease. The time from onset of symptoms to hospitalisation (and isolation) ranged between 0 and 10 days with a mean of 3.7 days (reported for 29 cases). The mean number of days to hospitalisation was 2.5 days for cases imported from China, but 4.6 days for those infected in Europe. This was mostly a result of delays in identifying the index cases of the two clusters in France and Germany. In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six took only a mean of 2 days to be hospitalised.
Symptoms at the point of diagnosis were reported for 31 cases. Two cases were asymptomatic and remained so until tested negative. The asymptomatic cases were tested as part of screening following repatriation and during contact tracing respectively. Of the remaining 29, 20 reported fever, 14 reported cough and eight reported weakness. Additional symptoms reported included headaches (6 cases), sore throat (2), rhinorrhoea (2), shortness of breath (2), myalgia (1), diarrhoea (1) and nausea (1). Fever was reported as the sole symptom for nine cases. In 16 of 29 symptomatic cases, the symptoms at diagnosis were consistent with the case definition for acute respiratory infection [16] , although it is possible that cases presented additional symptoms after diagnosis and these were not reported.
Data on pre-existing conditions were reported for seven cases; five had no pre-existing conditions while one was reported to be obese and one had pre-existing cardiac disease. No data on clinical signs e.g. dyspnea etc. were reported for any of the 38 cases.
All hospitalised cases had a benign clinical evolution except four, two reported in Italy and two reported in France, all of whom developed viral pneumonia. All three cases who were aged 65 years or over were admitted to intensive care and required respiratory support and one French case died. The case who died was hospitalised for 21 days and required intensive care and mechanical ventilation for 19 days. The duration of hospitalisation was reported for 16 cases with a median of 13 days (range: 8-23 days). As at 21 February 2020, four cases were still hospitalised.
All cases were confirmed according to specific assays targeting at least two separate genes (envelope (E) gene as a screening test and RNA-dependent RNA polymerase (RdRp) gene or nucleoprotein (N) gene for confirmation) [8, 17] . The specimen types tested were reported for 27 cases: 15 had positive nasopharyngeal swabs, nine had positive throat swabs, three cases had positive sputum, two had a positive nasal swab, one case had a positive nasopharyngeal aspirate and one a positive endotracheal aspirate.
As at 09:00 on 21 February, few COVID-19 cases had been detected in Europe compared with Asia. However the situation is rapidly developing, with a large outbreak recently identified in northern Italy, with transmission in several municipalities and at least two deaths [18] . As at 5 March 2020, there are 4,250 cases including 113 deaths reported among 38 countries in the WHO European region [19] .
In our analysis of early cases, we observed transmission in two broad contexts: sporadic cases among travellers from China (14 cases) and cases who acquired infection due to subsequent local transmission in Europe (21 cases). Our analysis shows that the time from symptom onset to hospitalisation/case isolation was about 3 days longer for locally acquired cases than for imported cases. People returning from affected areas are likely to have a low threshold to seek care and be tested when symptomatic, however delays in identifying the index cases of the two clusters in France and Germany meant that locally acquired cases took longer to be detected and isolated. Once the exposure is determined and contacts identified and quarantined (171 contacts in France and 200 in Germany for the clusters in Haute-Savoie and Bavaria, respectively), further cases are likely to be rapidly detected and isolated when they develop symptoms [15, 20] . In the German cluster, for example, the first three cases detected locally were hospitalised in a mean of 5.7 days, whereas the following six were hospitalised after a mean of 2 days. Locally acquired cases require significant resources for contact tracing and quarantine, and countries should be prepared to allocate considerable public health resources during the containment phase, should local clusters emerge in their population. In addition, prompt sharing of information on cases and contacts through international notification systems such as the International Health Regulations (IHR) mechanism and the European Commission's European Early Warning and Response System is essential to contain international spread of infection.
All of the imported cases had a history of travel to China. This was consistent with the epidemiological situation in Asia, and supported the recommendation for testing of suspected cases with travel history to China and potentially other areas of presumed ongoing community transmission. The situation has evolved rapidly since then, however, and the number of countries reporting COVID-19 transmission increased rapidly, notably with a large outbreak in northern Italy with 3,089 cases reported as at 5 March [18, 19] . Testing of suspected cases based on geographical risk of importation needs to be complemented with additional approaches to ensure early detection of local circulation of COVID-19, including through testing of severe acute respiratory infections in hospitals irrespectively of travel history as recommended in the WHO case definition updated on 27 February 2020 [21] .
The clinical presentation observed in the cases in Europe is that of an acute respiratory infection. However, of the 31 cases with information on symptoms, 20 cases presented with fever and nine cases presented only with fever and no other symptoms. These findings, which are consistent with other published case series, have prompted ECDC to include fever among several clinical signs or symptoms indicative for the suspected case definition.
Three cases were aged 65 years or over. All required admission to intensive care and were tourists (imported cases). These findings could reflect the average older age of the tourist population compared with the local contacts exposed to infection in Europe and do not allow us to draw any conclusion on the proportion of severe cases that we could expect in the general population of Europe. Despite this, the finding of older individuals being at higher risk of a severe clinical course is consistent with the evidence from Chinese case series published so far although the majority of infections in China have been mild [22, 23] .
This preliminary analysis is based on the first reported cases of COVID-19 cases in the WHO European Region. Given the small sample size, and limited completeness for some variables, all the results presented should be interpreted with caution.
With increasing numbers of cases in Europe, data from surveillance and investigations in the region can build on the evidence from countries in Asia experiencing more widespread transmission particularly on disease spectrum and the proportion of infections with severe outcome [22] . Understanding the infection-severity is critical to help plan for the impact on the healthcare system and the wider population. Serological studies are vital to understand the proportion of cases who are asymptomatic. Hospital-based surveillance could help estimate the incidence of severe cases and identify risk factors for severity and death. Established hospital surveillance systems that are in place for influenza and other diseases in Europe may be expanded for this purpose. In addition, a number of countries in Europe are adapting and, in some cases, already using existing sentinel primary care based surveillance systems for influenza to detect community transmission of SARS-CoV-2. This approach will be used globally to help identify evidence of widespread community transmission and, should the virus spread and containment no longer be deemed feasible, to monitor intensity of disease transmission, trends and its geographical spread.
Additional research is needed to complement surveillance data to build knowledge on the infectious period, modes of transmission, basic and effective reproduction numbers, and effectiveness of prevention and case management options also in settings outside of China. Such special studies are being conducted globally, including a cohort study on citizens repatriated from China to Europe, with the aim to extrapolate disease incidence and risk factors for infection in areas with community transmission. Countries together with ECDC and WHO, should use all opportunities to address these questions in a coordinated fashion at the European and global level.
provided input to the outline, multiple versions of the manuscript and gave approval to the final draft. | Why is additional research needed? | {
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" to complement surveillance data to build knowledge on the infectious period, modes of transmission, basic and effective reproduction numbers, and effectiveness of prevention and case management options also in settings outside of China."
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4,123 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What growing dysjunction has been witnessed? | {
"answer_start": [
292
],
"text": [
"a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. "
]
} | false |
4,124 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What is aiming to incorporate pathways to translation at the earliest stages? | {
"answer_start": [
502
],
"text": [
" recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses"
]
} | false |
4,125 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | How much have the number of biomedical research publications targeting 'translational' concepts has increased ? | {
"answer_start": [
842
],
"text": [
"exponentially, up 1800%"
]
} | false |
4,126 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What ways to solve the issues are outlined? | {
"answer_start": [
2042
],
"text": [
"by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. "
]
} | false |
4,127 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | How do these exact processes ultimately restrict viral infectivity? | {
"answer_start": [
2302
],
"text": [
" by strongly limiting virus genome sizes and their incorporation of new information. "
]
} | false |
4,128 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What does the author coin this evolutionary dilemma as? | {
"answer_start": [
2424
],
"text": [
"'information economy paradox'."
]
} | false |
4,129 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | How do many viruses resolve this ? | {
"answer_start": [
2492
],
"text": [
"by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost."
]
} | false |
4,130 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | How may this "Achilles Heel" be safely targeted? | {
"answer_start": [
2725
],
"text": [
" via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection."
]
} | false |
4,131 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | Why may MMHP-targeting therapies exhibit both robust and broadspectrum antiviral efficacy? | {
"answer_start": [
2852
],
"text": [
"since MMHPs are often conserved targets within and between virus families,"
]
} | false |
4,132 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What will achieving this through drug repurposing do? | {
"answer_start": [
3058
],
"text": [
" break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. "
]
} | false |
4,133 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What are also discussed by the author? | {
"answer_start": [
3215
],
"text": [
" alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology."
]
} | false |
4,134 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What does the author anticipate international efforts will do? | {
"answer_start": [
3429
],
"text": [
" will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed."
]
} | false |
4,138 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What do pathogens do upon infection? | {
"answer_start": [
3688
],
"text": [
"stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. "
]
} | false |
4,139 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What is the flip side ? | {
"answer_start": [
3822
],
"text": [
"this same process also causes immunopathology when prolonged or deregulated."
]
} | false |
4,140 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What do RBPs do? | {
"answer_start": [
4022
],
"text": [
"post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications"
]
} | false |
4,141 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What is included in RBPs? | {
"answer_start": [
4182
],
"text": [
"tristetraprolin and AUF1"
]
} | false |
4,142 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What do tristetraprolin and AUF1, do? | {
"answer_start": [
4214
],
"text": [
"promote degradation of AU-rich element (ARE)-containing mRNA"
]
} | false |
4,143 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What do RBPs include? | {
"answer_start": [
4276
],
"text": [
"members of the Roquin and Regnase families"
]
} | false |
4,144 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What domembers of the Roquin and Regnase families do? | {
"answer_start": [
4339
],
"text": [
"promote or effect degradation of mRNAs harbouring stem-loop structures"
]
} | false |
4,145 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What do the RBPs include? | {
"answer_start": [
4453
],
"text": [
"RNA methylation machinery"
]
} | false |
4,146 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What is the increasingly apparent role of RNA methylation machinery ? | {
"answer_start": [
4479
],
"text": [
"in controlling inflammatory mRNA stability."
]
} | false |
4,147 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | Where do these activities take place? | {
"answer_start": [
4550
],
"text": [
" in various subcellular compartments "
]
} | false |
4,148 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What happens to these activities during infection? | {
"answer_start": [
4591
],
"text": [
"are differentially regulated"
]
} | false |
4,149 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | In this way, what do the mRNA-destabilising RBPs constitute ? | {
"answer_start": [
4686
],
"text": [
"a 'brake' on the immune system"
]
} | false |
4,150 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What can be done with the 'brake' on the immune system? | {
"answer_start": [
4724
],
"text": [
"may ultimately be toggled therapeutically"
]
} | false |
4,151 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What does the author anticipate that continued efforts will lead to? | {
"answer_start": [
5011
],
"text": [
"Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin"
]
} | false |
4,152 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What is another mRNA under post-transcriptional regulation by Regnase-1 and Roquin? | {
"answer_start": [
5088
],
"text": [
" Furin"
]
} | false |
4,153 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What does Furin encode? | {
"answer_start": [
5110
],
"text": [
"a conserved proprotein convertase crucial in human health and disease."
]
} | false |
4,154 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What are Furin, along with other PCSK family members implicated in? | {
"answer_start": [
5246
],
"text": [
" in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV)"
]
} | false |
4,155 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What do Braun and Sauter review? | {
"answer_start": [
5516
],
"text": [
" the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics."
]
} | false |
4,156 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What dis their recent work reveal? | {
"answer_start": [
5671
],
"text": [
"how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity."
]
} | false |
4,157 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What has the increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies led to? | {
"answer_start": [
6119
],
"text": [
" a recent boom in metagenomics and the cataloguing of the microbiome of our world."
]
} | false |
4,160 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What was this system used for the first time for? | {
"answer_start": [
6678
],
"text": [
" to directly sequence an RNA virus genome (IAV)"
]
} | false |
4,161 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What have decades of basic immunology research provided ? | {
"answer_start": [
7152
],
"text": [
"a near-complete picture of the main armaments in the human antiviral arsenal. "
]
} | false |
4,162 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What has this focus on mammalian defences and pathologies sidelined? | {
"answer_start": [
7307
],
"text": [
"examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere."
]
} | false |
4,163 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What has CRISPR/Cas antiviral immune system of prokaryotes been repurposed as? | {
"answer_start": [
7513
],
"text": [
"as a revolutionary gene-editing biotechnology in plants and animals."
]
} | false |
4,164 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What is another case in point? | {
"answer_start": [
7596
],
"text": [
"the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs)"
]
} | false |
4,165 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What are the ancient lineage of NCLDVs?
| {
"answer_start": [
7673
],
"text": [
"emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions."
]
} | false |
4,166 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What do the recent efforts indicate regarding hundreds of human and avian infectious viruses? | {
"answer_start": [
7949
],
"text": [
" the true number may be in the millions and many harbour zoonotic potential. "
]
} | false |
4,167 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What is neo-virology? | {
"answer_start": [
8235
],
"text": [
" an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium."
]
} | false |
4,168 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What is predicted these efforts on neo-virology will unlock? | {
"answer_start": [
8382
],
"text": [
"a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution."
]
} | false |
4,169 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What are the two of the four pillars of the National Innovation and Science Agenda? | {
"answer_start": [
8965
],
"text": [
"Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region"
]
} | false |
4,170 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What do Australia's Medical Research and Innovation Priorities include? | {
"answer_start": [
9226
],
"text": [
"antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure,"
]
} | false |
4,171 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What is essential for these priority outcomes? | {
"answer_start": [
9414
],
"text": [
"Establishing durable international relationships that integrate diverse expertise"
]
} | false |
4,172 | Frontiers in antiviral therapy and immunotherapy
https://doi.org/10.1002/cti2.1115
SHA: facbfdfa7189ca9ff83dc30e5d241ab22e962dbf
Authors: Heaton, Steven M
Date: 2020
DOI: 10.1002/cti2.1115
License: cc-by
Abstract: nan
Text: Globally, recent decades have witnessed a growing disjunction, a 'Valley of Death' 1,2 no less, between broadening strides in fundamental biomedical research and their incommensurate reach into the clinic. Plumbing work on research funding and development pipelines through recent changes in the structure of government funding, 2 new public and private joint ventures and specialist undergraduate and postgraduate courses now aim to incorporate pathways to translation at the earliest stages. Reflecting this shift, the number of biomedical research publications targeting 'translational' concepts has increased exponentially, up 1800% between 2003 and 2014 3 and continuing to rise rapidly up to the present day. Fuelled by the availability of new research technologies, as well as changing disease, cost and other pressing issues of our time, further growth in this exciting space will undoubtedly continue. Despite recent advances in the therapeutic control of immune function and viral infection, current therapies are often challenging to develop, expensive to deploy and readily select for resistance-conferring mutants. Shaped by the hostvirus immunological 'arms race' and tempered in the forge of deep time, the biodiversity of our world is increasingly being harnessed for new biotechnologies and therapeutics. Simultaneously, a shift towards host-oriented antiviral therapies is currently underway. In this Clinical & Translational Immunology Special Feature, I illustrate a strategic vision integrating these themes to create new, effective, economical and robust antiviral therapies and immunotherapies, with both the realities and the opportunities afforded to researchers working in our changing world squarely in mind.
Opening this CTI Special Feature, I outline ways these issues may be solved by creatively leveraging the so-called 'strengths' of viruses. Viral RNA polymerisation and reverse transcription enable resistance to treatment by conferring extraordinary genetic diversity. However, these exact processes ultimately restrict viral infectivity by strongly limiting virus genome sizes and their incorporation of new information. I coin this evolutionary dilemma the 'information economy paradox'. Many viruses attempt to resolve this by manipulating multifunctional or multitasking host cell proteins (MMHPs), thereby maximising host subversion and viral infectivity at minimal informational cost. 4 I argue this exposes an 'Achilles Heel' that may be safely targeted via host-oriented therapies to impose devastating informational and fitness barriers on escape mutant selection. Furthermore, since MMHPs are often conserved targets within and between virus families, MMHP-targeting therapies may exhibit both robust and broadspectrum antiviral efficacy. Achieving this through drug repurposing will break the vicious cycle of escalating therapeutic development costs and trivial escape mutant selection, both quickly and in multiple places. I also discuss alternative posttranslational and RNA-based antiviral approaches, designer vaccines, immunotherapy and the emerging field of neo-virology. 4 I anticipate international efforts in these areas over the coming decade will enable the tapping of useful new biological functions and processes, methods for controlling infection, and the deployment of symbiotic or subclinical viruses in new therapies and biotechnologies that are so crucially needed.
Upon infection, pathogens stimulate expression of numerous host inflammatory factors that support recruitment and activation of immune cells. On the flip side, this same process also causes immunopathology when prolonged or deregulated. 5 In their contribution to this Special Feature, Yoshinaga and Takeuchi review endogenous RNA-binding proteins (RBPs) that post-transcriptionally control expression of crucial inflammatory factors in various tissues and their potential therapeutic applications. 6 These RBPs include tristetraprolin and AUF1, which promote degradation of AU-rich element (ARE)-containing mRNA; members of the Roquin and Regnase families, which respectively promote or effect degradation of mRNAs harbouring stem-loop structures; and the increasingly apparent role of the RNA methylation machinery in controlling inflammatory mRNA stability. These activities take place in various subcellular compartments and are differentially regulated during infection. In this way, mRNA-destabilising RBPs constitute a 'brake' on the immune system, which may ultimately be toggled therapeutically. I anticipate continued efforts in this area will lead to new methods of regaining control over inflammation in autoimmunity, selectively enhancing immunity in immunotherapy, and modulating RNA synthesis and virus replication during infection.
Another mRNA under post-transcriptional regulation by Regnase-1 and Roquin is Furin, which encodes a conserved proprotein convertase crucial in human health and disease. Furin, along with other PCSK family members, is widely implicated in immune regulation, cancer and the entry, maturation or release of a broad array of evolutionarily diverse viruses including human papillomavirus (HPV), influenza (IAV), Ebola (EboV), dengue (DenV) and human immunodeficiency virus (HIV). Here, Braun and Sauter review the roles of furin in these processes, as well as the history and future of furin-targeting therapeutics. 7 They also discuss their recent work revealing how two IFN-cinducible factors exhibit broad-spectrum inhibition of IAV, measles (MV), zika (ZikV) and HIV by suppressing furin activity. 8 Over the coming decade, I expect to see an ever-finer spatiotemporal resolution of host-oriented therapies to achieve safe, effective and broad-spectrum yet costeffective therapies for clinical use.
The increasing abundance of affordable, sensitive, high-throughput genome sequencing technologies has led to a recent boom in metagenomics and the cataloguing of the microbiome of our world. The MinION nanopore sequencer is one of the latest innovations in this space, enabling direct sequencing in a miniature form factor with only minimal sample preparation and a consumer-grade laptop computer. Nakagawa and colleagues here report on their latest experiments using this system, further improving its performance for use in resource-poor contexts for meningitis diagnoses. 9 While direct sequencing of viral genomic RNA is challenging, this system was recently used to directly sequence an RNA virus genome (IAV) for the first time. 10 I anticipate further improvements in the performance of such devices over the coming decade will transform virus surveillance efforts, the importance of which was underscored by the recent EboV and novel coronavirus (nCoV / COVID-19) outbreaks, enabling rapid deployment of antiviral treatments that take resistance-conferring mutations into account.
Decades of basic immunology research have provided a near-complete picture of the main armaments in the human antiviral arsenal. Nevertheless, this focus on mammalian defences and pathologies has sidelined examination of the types and roles of viruses and antiviral defences that exist throughout our biosphere. One case in point is the CRISPR/Cas antiviral immune system of prokaryotes, which is now repurposed as a revolutionary gene-editing biotechnology in plants and animals. 11 Another is the ancient lineage of nucleocytosolic large DNA viruses (NCLDVs), which are emerging human pathogens that possess enormous genomes of up to several megabases in size encoding hundreds of proteins with unique and unknown functions. 12 Moreover, hundreds of human-and avian-infective viruses such as IAV strain H5N1 are known, but recent efforts indicate the true number may be in the millions and many harbour zoonotic potential. 13 It is increasingly clear that host-virus interactions have generated truly vast yet poorly understood and untapped biodiversity. Closing this Special Feature, Watanabe and Kawaoka elaborate on neo-virology, an emerging field engaged in cataloguing and characterising this biodiversity through a global consortium. 14 I predict these efforts will unlock a vast wealth of currently unexplored biodiversity, leading to biotechnologies and treatments that leverage the host-virus interactions developed throughout evolution.
When biomedical innovations fall into the 'Valley of Death', patients who are therefore not reached all too often fall with them. Being entrusted with the resources and expectation to conceive, deliver and communicate dividends to society is both cherished and eagerly pursued at every stage of our careers. Nevertheless, the road to research translation is winding and is built on a foundation of basic research. Supporting industry-academia collaboration and nurturing talent and skills in the Indo-Pacific region are two of the four pillars of the National Innovation and Science Agenda. 2 These frame Australia's Medical Research and Innovation Priorities, which include antimicrobial resistance, global health and health security, drug repurposing and translational research infrastructure, 15 capturing many of the key elements of this CTI Special Feature. Establishing durable international relationships that integrate diverse expertise is essential to delivering these outcomes. To this end, NHMRC has recently taken steps under the International Engagement Strategy 16 to increase cooperation with its counterparts overseas. These include the Japan Agency for Medical Research and Development (AMED), tasked with translating the biomedical research output of that country. Given the reciprocal efforts at accelerating bilateral engagement currently underway, 17 the prospects for new areas of international cooperation and mobility have never been more exciting nor urgent. With the above in mind, all contributions to this CTI Special Feature I have selected from research presented by fellow invitees to the 2018 Awaji International Forum on Infection and Immunity (AIFII) and 2017 Consortium of Biological Sciences (ConBio) conferences in Japan. Both Australia and Japan have strong traditions in immunology and related disciplines, and I predict that the quantity, quality and importance of our bilateral cooperation will accelerate rapidly over the short to medium term. By expanding and cooperatively leveraging our respective research strengths, our efforts may yet solve the many pressing disease, cost and other sustainability issues of our time. | What is the Japan AMED tasked with? | {
"answer_start": [
9774
],
"text": [
"translating the biomedical research output of that country."
]
} | false |
3,847 | Haunted with and hunting for viruses
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7089303/
SHA: c51c4f6146d0c636bc4dc3839c16b9e3ef52849a
Authors: Gao, George Fu; Wu, Ying
Date: 2013-08-07
DOI: 10.1007/s11427-013-4525-x
License: cc-by
Abstract: nan
Text: pecially with next-generation sequencing (NGS) for new virus genome discovery, e.g., Ruben Donis et al. [10] sequenced a bat-derived influenza virus genome by using NGS in 2012, raising a serious question as to whether or not our seasonal or pandemic flu might have another reservoir host. Chen and colleagues [11] confirmed the SFTSV independently by using NGS. Indeed, metagenomics analysis has yielded a great deal of new viruses, especially from the environment. Our actively hunting for new viruses has made some significant contributions for our understanding of virus ecology, pathogenesis and interspecies transmission.
Science China Life Sciences has focused on this hot topic in the event of the H7N9 outbreak after a comprehensive overview of the topic addressing HPAIV H5N1 in 2009 in the journal [12] [13] [14] . In this issue, six groups have been invited to present their recent findings on the emerging viruses, in addition to a previous report on H7N9 [3] .
Shi [15] reviewed recent discoveries of new viruses or virus genomes from bat. Bat is believed to harbor many more viruses than we ever thought as a reservoir host or even a susceptible host [16] . After the SARS-CoV virus, we have been actively seeking for new coronaviruses from bat and have yielded many of them, including potential human infecting HKU-1, 4, 5 and 9 [17, 18] . Recent MERS-CoV infection is another example for severe disease caused by used-to-be-less pathogenic coronaviruses. Shi and colleagues [19] by using NGS have discovered many unknown animal viruses from bat, especially some important paramyxoviruses and reoviruses. Filovirus has also been identified in bat with potential severe outcomes. Lyssaviruses (with many genotypes, including rabies virus) in the Rhabdoviridae family have been linked with severe fatal human cases, even in the developed countries, including Australia, with the bites of bats in the city [20, 21] . The potential roles of these viruses in bats for interspecies transmission are yet to be elucidated.
Tan and colleagues [22] specifically focused on the newly-emerged MERS-CoV. The virus was identified in 2012 in the Middle East with some exported cases to Europe. In 2013 the virus has been re-emerging and expanding its borders to more European countries. In the initial diagnosis, the pan-coronavirus real-time reverse transcription polymerase chain reaction (RT-PCR) assay played a very important role for the identification of the causative agents. By using this method, scientists detected an expected-size PCR fragment for the corresponding conserved region of ORF1b of the replicase gene of a coronavirus. This is another example that molecular biology methods played for the discovery of new pathogens. Soon the receptor used by MERS-CoV to enter the host cells was identified [23] and the molecular basis of the receptor binding to the virus was also elucidated recently [8] .
Enterovirus has been known as serious human pathogens for a long time but their significance to the public health has been emphasized by the emergence of enterovirus 71 in 1998 as a serious pathogenic agents for children in Taiwan [24] and re-emerged in mainland China in 2008 [25] . In this issue, Duan and colleagues [26] summarized the findings of new enteroviruses by using NGS. Because of the application of new NGS technology they also challenged the Koch's postulates. A new model of Koch's postulates, named the metagenomic Koch's postulates, has provided guidance for the study of the pathogenicity of novel viruses. The review also provided a detailed description of the NGS and related molecular methods for the virus discovery followed by a list of new enteroviruses found in human feces. These include viruses in the family of Piconaviridae, Parvoviridae, Circoviridae, Astroviridae and Polyomaviridae.
Yu Xue-Jie and colleagues [27] reviewed the new bunyavirus, SFTSV, identified in China. As the virus discoverers, they have overviewed the whole process of the discovery, which is helpful and meaningful for the new virus discoveries in the future. The disease caused by SFTSV, with a CFR of 12%, had been in China for a couple of years before the causative agent was finally identified. There are still a lot of questions remained unknown for this new virus and vigorous studies are in great need. The transmission route of the virus has not been clarified but tick as vector is suspected. Domestic and wild animals, e.g., goats, boars, cattle and dogs, are believed to be the virus-amplifying hosts. Therefore the effective control measures are still under evaluation. Vaccines protecting the SFTSV infection are under its way in Chinese Center for Disease Control and Prevention. Recently a similar virus has been identified in both Japan and USA (a new name of Heartland virus was proposed for the US virus) [9] .
In addition to new viruses infecting human beings, some new viruses infecting animals but their public health significance needing to be further evaluated, have also been discovered. The new flavivirus, duck egg-drop syndrome virus (DEDSV), is a good example. Su and colleagues [28] reviewed the characterization of the DEDSV and its disease form in this issue. The virus was found closely-related to a long-time-known virus, Tembusu virus [29, 30] . Initially, the disease was only found in egg-raising ducks but soon it was found in pigeons, chickens and geese [31, 32] . Yet the transmission vector, though mosquitoes are suspected, has not been identified. Due to the public health concerns of its related viruses, potential human infection of DEDSV should be evaluated.
Research on insect viruses is reviving in recent years. In this issue, Zhou and colleagues [33] reviewed the newly-identified insect viruses in China. Insects are the largest group of animals on the Earth therefore they also carry many more viruses. Studies on these viruses can provide useful knowledge for our understanding about animal or human infecting viruses. More importantly, modification and application of insect-infecting viruses can be used as effective biologicals for the control of insect pest. The new viruses identified include Wuhan nodavirus (WhNV), a member of family Nodaviridae; Dendrolimus punctatus tetravirus (DpTV), a new member of the genus Omegatetravirus of the family Alphatetravirida; Ectropis obliqua picorna-like virus (EoV), a positive-strand RNA virus causing a lethal granulosis infection in the larvae of the tea looper (Ectropis obliqua), the virus a member of the Flaviridae family.
While we are enjoying ourselves with the civilization of modern societies, the ecology has ever been changing. Human beings encounter more ecology-climate-changing problems, including the zoonotic pathogens. We have to face some unknown pathogenic agents passively. To get ourselves well prepared we also ought to actively hunt for unknown pathogens. Prediction and pre-warning can only be realized by knowing more about the unknown. This is especially true for infectious agents. | What serious question was raised? | {
"answer_start": [
464
],
"text": [
"as to whether or not our seasonal or pandemic flu might have another reservoir host."
]
} | false |
3,848 | Haunted with and hunting for viruses
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7089303/
SHA: c51c4f6146d0c636bc4dc3839c16b9e3ef52849a
Authors: Gao, George Fu; Wu, Ying
Date: 2013-08-07
DOI: 10.1007/s11427-013-4525-x
License: cc-by
Abstract: nan
Text: pecially with next-generation sequencing (NGS) for new virus genome discovery, e.g., Ruben Donis et al. [10] sequenced a bat-derived influenza virus genome by using NGS in 2012, raising a serious question as to whether or not our seasonal or pandemic flu might have another reservoir host. Chen and colleagues [11] confirmed the SFTSV independently by using NGS. Indeed, metagenomics analysis has yielded a great deal of new viruses, especially from the environment. Our actively hunting for new viruses has made some significant contributions for our understanding of virus ecology, pathogenesis and interspecies transmission.
Science China Life Sciences has focused on this hot topic in the event of the H7N9 outbreak after a comprehensive overview of the topic addressing HPAIV H5N1 in 2009 in the journal [12] [13] [14] . In this issue, six groups have been invited to present their recent findings on the emerging viruses, in addition to a previous report on H7N9 [3] .
Shi [15] reviewed recent discoveries of new viruses or virus genomes from bat. Bat is believed to harbor many more viruses than we ever thought as a reservoir host or even a susceptible host [16] . After the SARS-CoV virus, we have been actively seeking for new coronaviruses from bat and have yielded many of them, including potential human infecting HKU-1, 4, 5 and 9 [17, 18] . Recent MERS-CoV infection is another example for severe disease caused by used-to-be-less pathogenic coronaviruses. Shi and colleagues [19] by using NGS have discovered many unknown animal viruses from bat, especially some important paramyxoviruses and reoviruses. Filovirus has also been identified in bat with potential severe outcomes. Lyssaviruses (with many genotypes, including rabies virus) in the Rhabdoviridae family have been linked with severe fatal human cases, even in the developed countries, including Australia, with the bites of bats in the city [20, 21] . The potential roles of these viruses in bats for interspecies transmission are yet to be elucidated.
Tan and colleagues [22] specifically focused on the newly-emerged MERS-CoV. The virus was identified in 2012 in the Middle East with some exported cases to Europe. In 2013 the virus has been re-emerging and expanding its borders to more European countries. In the initial diagnosis, the pan-coronavirus real-time reverse transcription polymerase chain reaction (RT-PCR) assay played a very important role for the identification of the causative agents. By using this method, scientists detected an expected-size PCR fragment for the corresponding conserved region of ORF1b of the replicase gene of a coronavirus. This is another example that molecular biology methods played for the discovery of new pathogens. Soon the receptor used by MERS-CoV to enter the host cells was identified [23] and the molecular basis of the receptor binding to the virus was also elucidated recently [8] .
Enterovirus has been known as serious human pathogens for a long time but their significance to the public health has been emphasized by the emergence of enterovirus 71 in 1998 as a serious pathogenic agents for children in Taiwan [24] and re-emerged in mainland China in 2008 [25] . In this issue, Duan and colleagues [26] summarized the findings of new enteroviruses by using NGS. Because of the application of new NGS technology they also challenged the Koch's postulates. A new model of Koch's postulates, named the metagenomic Koch's postulates, has provided guidance for the study of the pathogenicity of novel viruses. The review also provided a detailed description of the NGS and related molecular methods for the virus discovery followed by a list of new enteroviruses found in human feces. These include viruses in the family of Piconaviridae, Parvoviridae, Circoviridae, Astroviridae and Polyomaviridae.
Yu Xue-Jie and colleagues [27] reviewed the new bunyavirus, SFTSV, identified in China. As the virus discoverers, they have overviewed the whole process of the discovery, which is helpful and meaningful for the new virus discoveries in the future. The disease caused by SFTSV, with a CFR of 12%, had been in China for a couple of years before the causative agent was finally identified. There are still a lot of questions remained unknown for this new virus and vigorous studies are in great need. The transmission route of the virus has not been clarified but tick as vector is suspected. Domestic and wild animals, e.g., goats, boars, cattle and dogs, are believed to be the virus-amplifying hosts. Therefore the effective control measures are still under evaluation. Vaccines protecting the SFTSV infection are under its way in Chinese Center for Disease Control and Prevention. Recently a similar virus has been identified in both Japan and USA (a new name of Heartland virus was proposed for the US virus) [9] .
In addition to new viruses infecting human beings, some new viruses infecting animals but their public health significance needing to be further evaluated, have also been discovered. The new flavivirus, duck egg-drop syndrome virus (DEDSV), is a good example. Su and colleagues [28] reviewed the characterization of the DEDSV and its disease form in this issue. The virus was found closely-related to a long-time-known virus, Tembusu virus [29, 30] . Initially, the disease was only found in egg-raising ducks but soon it was found in pigeons, chickens and geese [31, 32] . Yet the transmission vector, though mosquitoes are suspected, has not been identified. Due to the public health concerns of its related viruses, potential human infection of DEDSV should be evaluated.
Research on insect viruses is reviving in recent years. In this issue, Zhou and colleagues [33] reviewed the newly-identified insect viruses in China. Insects are the largest group of animals on the Earth therefore they also carry many more viruses. Studies on these viruses can provide useful knowledge for our understanding about animal or human infecting viruses. More importantly, modification and application of insect-infecting viruses can be used as effective biologicals for the control of insect pest. The new viruses identified include Wuhan nodavirus (WhNV), a member of family Nodaviridae; Dendrolimus punctatus tetravirus (DpTV), a new member of the genus Omegatetravirus of the family Alphatetravirida; Ectropis obliqua picorna-like virus (EoV), a positive-strand RNA virus causing a lethal granulosis infection in the larvae of the tea looper (Ectropis obliqua), the virus a member of the Flaviridae family.
While we are enjoying ourselves with the civilization of modern societies, the ecology has ever been changing. Human beings encounter more ecology-climate-changing problems, including the zoonotic pathogens. We have to face some unknown pathogenic agents passively. To get ourselves well prepared we also ought to actively hunt for unknown pathogens. Prediction and pre-warning can only be realized by knowing more about the unknown. This is especially true for infectious agents. | What is a recent discovery? | {
"answer_start": [
1315
],
"text": [
"Bat is believed to harbor many more viruses than we ever thought as a reservoir host or even a susceptible host "
]
} | false |
3,849 | Haunted with and hunting for viruses
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7089303/
SHA: c51c4f6146d0c636bc4dc3839c16b9e3ef52849a
Authors: Gao, George Fu; Wu, Ying
Date: 2013-08-07
DOI: 10.1007/s11427-013-4525-x
License: cc-by
Abstract: nan
Text: pecially with next-generation sequencing (NGS) for new virus genome discovery, e.g., Ruben Donis et al. [10] sequenced a bat-derived influenza virus genome by using NGS in 2012, raising a serious question as to whether or not our seasonal or pandemic flu might have another reservoir host. Chen and colleagues [11] confirmed the SFTSV independently by using NGS. Indeed, metagenomics analysis has yielded a great deal of new viruses, especially from the environment. Our actively hunting for new viruses has made some significant contributions for our understanding of virus ecology, pathogenesis and interspecies transmission.
Science China Life Sciences has focused on this hot topic in the event of the H7N9 outbreak after a comprehensive overview of the topic addressing HPAIV H5N1 in 2009 in the journal [12] [13] [14] . In this issue, six groups have been invited to present their recent findings on the emerging viruses, in addition to a previous report on H7N9 [3] .
Shi [15] reviewed recent discoveries of new viruses or virus genomes from bat. Bat is believed to harbor many more viruses than we ever thought as a reservoir host or even a susceptible host [16] . After the SARS-CoV virus, we have been actively seeking for new coronaviruses from bat and have yielded many of them, including potential human infecting HKU-1, 4, 5 and 9 [17, 18] . Recent MERS-CoV infection is another example for severe disease caused by used-to-be-less pathogenic coronaviruses. Shi and colleagues [19] by using NGS have discovered many unknown animal viruses from bat, especially some important paramyxoviruses and reoviruses. Filovirus has also been identified in bat with potential severe outcomes. Lyssaviruses (with many genotypes, including rabies virus) in the Rhabdoviridae family have been linked with severe fatal human cases, even in the developed countries, including Australia, with the bites of bats in the city [20, 21] . The potential roles of these viruses in bats for interspecies transmission are yet to be elucidated.
Tan and colleagues [22] specifically focused on the newly-emerged MERS-CoV. The virus was identified in 2012 in the Middle East with some exported cases to Europe. In 2013 the virus has been re-emerging and expanding its borders to more European countries. In the initial diagnosis, the pan-coronavirus real-time reverse transcription polymerase chain reaction (RT-PCR) assay played a very important role for the identification of the causative agents. By using this method, scientists detected an expected-size PCR fragment for the corresponding conserved region of ORF1b of the replicase gene of a coronavirus. This is another example that molecular biology methods played for the discovery of new pathogens. Soon the receptor used by MERS-CoV to enter the host cells was identified [23] and the molecular basis of the receptor binding to the virus was also elucidated recently [8] .
Enterovirus has been known as serious human pathogens for a long time but their significance to the public health has been emphasized by the emergence of enterovirus 71 in 1998 as a serious pathogenic agents for children in Taiwan [24] and re-emerged in mainland China in 2008 [25] . In this issue, Duan and colleagues [26] summarized the findings of new enteroviruses by using NGS. Because of the application of new NGS technology they also challenged the Koch's postulates. A new model of Koch's postulates, named the metagenomic Koch's postulates, has provided guidance for the study of the pathogenicity of novel viruses. The review also provided a detailed description of the NGS and related molecular methods for the virus discovery followed by a list of new enteroviruses found in human feces. These include viruses in the family of Piconaviridae, Parvoviridae, Circoviridae, Astroviridae and Polyomaviridae.
Yu Xue-Jie and colleagues [27] reviewed the new bunyavirus, SFTSV, identified in China. As the virus discoverers, they have overviewed the whole process of the discovery, which is helpful and meaningful for the new virus discoveries in the future. The disease caused by SFTSV, with a CFR of 12%, had been in China for a couple of years before the causative agent was finally identified. There are still a lot of questions remained unknown for this new virus and vigorous studies are in great need. The transmission route of the virus has not been clarified but tick as vector is suspected. Domestic and wild animals, e.g., goats, boars, cattle and dogs, are believed to be the virus-amplifying hosts. Therefore the effective control measures are still under evaluation. Vaccines protecting the SFTSV infection are under its way in Chinese Center for Disease Control and Prevention. Recently a similar virus has been identified in both Japan and USA (a new name of Heartland virus was proposed for the US virus) [9] .
In addition to new viruses infecting human beings, some new viruses infecting animals but their public health significance needing to be further evaluated, have also been discovered. The new flavivirus, duck egg-drop syndrome virus (DEDSV), is a good example. Su and colleagues [28] reviewed the characterization of the DEDSV and its disease form in this issue. The virus was found closely-related to a long-time-known virus, Tembusu virus [29, 30] . Initially, the disease was only found in egg-raising ducks but soon it was found in pigeons, chickens and geese [31, 32] . Yet the transmission vector, though mosquitoes are suspected, has not been identified. Due to the public health concerns of its related viruses, potential human infection of DEDSV should be evaluated.
Research on insect viruses is reviving in recent years. In this issue, Zhou and colleagues [33] reviewed the newly-identified insect viruses in China. Insects are the largest group of animals on the Earth therefore they also carry many more viruses. Studies on these viruses can provide useful knowledge for our understanding about animal or human infecting viruses. More importantly, modification and application of insect-infecting viruses can be used as effective biologicals for the control of insect pest. The new viruses identified include Wuhan nodavirus (WhNV), a member of family Nodaviridae; Dendrolimus punctatus tetravirus (DpTV), a new member of the genus Omegatetravirus of the family Alphatetravirida; Ectropis obliqua picorna-like virus (EoV), a positive-strand RNA virus causing a lethal granulosis infection in the larvae of the tea looper (Ectropis obliqua), the virus a member of the Flaviridae family.
While we are enjoying ourselves with the civilization of modern societies, the ecology has ever been changing. Human beings encounter more ecology-climate-changing problems, including the zoonotic pathogens. We have to face some unknown pathogenic agents passively. To get ourselves well prepared we also ought to actively hunt for unknown pathogens. Prediction and pre-warning can only be realized by knowing more about the unknown. This is especially true for infectious agents. | Which bat virus have been found to be linked with diseases? | {
"answer_start": [
1562
],
"text": [
"potential human infecting HKU-1, 4, 5 and 9 [17, 18] . Recent MERS-CoV infection is another example for severe disease caused by used-to-be-less pathogenic coronaviruses. Shi and colleagues [19] by using NGS have discovered many unknown animal viruses from bat, especially some important paramyxoviruses and reoviruses. Filovirus has also been identified in bat with potential severe outcomes. Lyssaviruses (with many genotypes, including rabies virus) in the Rhabdoviridae family have been linked with severe fatal human cases, even in the developed countries, including Australia, with the bites of bats in the city [20, 21] ."
]
} | false |
3,850 | Haunted with and hunting for viruses
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7089303/
SHA: c51c4f6146d0c636bc4dc3839c16b9e3ef52849a
Authors: Gao, George Fu; Wu, Ying
Date: 2013-08-07
DOI: 10.1007/s11427-013-4525-x
License: cc-by
Abstract: nan
Text: pecially with next-generation sequencing (NGS) for new virus genome discovery, e.g., Ruben Donis et al. [10] sequenced a bat-derived influenza virus genome by using NGS in 2012, raising a serious question as to whether or not our seasonal or pandemic flu might have another reservoir host. Chen and colleagues [11] confirmed the SFTSV independently by using NGS. Indeed, metagenomics analysis has yielded a great deal of new viruses, especially from the environment. Our actively hunting for new viruses has made some significant contributions for our understanding of virus ecology, pathogenesis and interspecies transmission.
Science China Life Sciences has focused on this hot topic in the event of the H7N9 outbreak after a comprehensive overview of the topic addressing HPAIV H5N1 in 2009 in the journal [12] [13] [14] . In this issue, six groups have been invited to present their recent findings on the emerging viruses, in addition to a previous report on H7N9 [3] .
Shi [15] reviewed recent discoveries of new viruses or virus genomes from bat. Bat is believed to harbor many more viruses than we ever thought as a reservoir host or even a susceptible host [16] . After the SARS-CoV virus, we have been actively seeking for new coronaviruses from bat and have yielded many of them, including potential human infecting HKU-1, 4, 5 and 9 [17, 18] . Recent MERS-CoV infection is another example for severe disease caused by used-to-be-less pathogenic coronaviruses. Shi and colleagues [19] by using NGS have discovered many unknown animal viruses from bat, especially some important paramyxoviruses and reoviruses. Filovirus has also been identified in bat with potential severe outcomes. Lyssaviruses (with many genotypes, including rabies virus) in the Rhabdoviridae family have been linked with severe fatal human cases, even in the developed countries, including Australia, with the bites of bats in the city [20, 21] . The potential roles of these viruses in bats for interspecies transmission are yet to be elucidated.
Tan and colleagues [22] specifically focused on the newly-emerged MERS-CoV. The virus was identified in 2012 in the Middle East with some exported cases to Europe. In 2013 the virus has been re-emerging and expanding its borders to more European countries. In the initial diagnosis, the pan-coronavirus real-time reverse transcription polymerase chain reaction (RT-PCR) assay played a very important role for the identification of the causative agents. By using this method, scientists detected an expected-size PCR fragment for the corresponding conserved region of ORF1b of the replicase gene of a coronavirus. This is another example that molecular biology methods played for the discovery of new pathogens. Soon the receptor used by MERS-CoV to enter the host cells was identified [23] and the molecular basis of the receptor binding to the virus was also elucidated recently [8] .
Enterovirus has been known as serious human pathogens for a long time but their significance to the public health has been emphasized by the emergence of enterovirus 71 in 1998 as a serious pathogenic agents for children in Taiwan [24] and re-emerged in mainland China in 2008 [25] . In this issue, Duan and colleagues [26] summarized the findings of new enteroviruses by using NGS. Because of the application of new NGS technology they also challenged the Koch's postulates. A new model of Koch's postulates, named the metagenomic Koch's postulates, has provided guidance for the study of the pathogenicity of novel viruses. The review also provided a detailed description of the NGS and related molecular methods for the virus discovery followed by a list of new enteroviruses found in human feces. These include viruses in the family of Piconaviridae, Parvoviridae, Circoviridae, Astroviridae and Polyomaviridae.
Yu Xue-Jie and colleagues [27] reviewed the new bunyavirus, SFTSV, identified in China. As the virus discoverers, they have overviewed the whole process of the discovery, which is helpful and meaningful for the new virus discoveries in the future. The disease caused by SFTSV, with a CFR of 12%, had been in China for a couple of years before the causative agent was finally identified. There are still a lot of questions remained unknown for this new virus and vigorous studies are in great need. The transmission route of the virus has not been clarified but tick as vector is suspected. Domestic and wild animals, e.g., goats, boars, cattle and dogs, are believed to be the virus-amplifying hosts. Therefore the effective control measures are still under evaluation. Vaccines protecting the SFTSV infection are under its way in Chinese Center for Disease Control and Prevention. Recently a similar virus has been identified in both Japan and USA (a new name of Heartland virus was proposed for the US virus) [9] .
In addition to new viruses infecting human beings, some new viruses infecting animals but their public health significance needing to be further evaluated, have also been discovered. The new flavivirus, duck egg-drop syndrome virus (DEDSV), is a good example. Su and colleagues [28] reviewed the characterization of the DEDSV and its disease form in this issue. The virus was found closely-related to a long-time-known virus, Tembusu virus [29, 30] . Initially, the disease was only found in egg-raising ducks but soon it was found in pigeons, chickens and geese [31, 32] . Yet the transmission vector, though mosquitoes are suspected, has not been identified. Due to the public health concerns of its related viruses, potential human infection of DEDSV should be evaluated.
Research on insect viruses is reviving in recent years. In this issue, Zhou and colleagues [33] reviewed the newly-identified insect viruses in China. Insects are the largest group of animals on the Earth therefore they also carry many more viruses. Studies on these viruses can provide useful knowledge for our understanding about animal or human infecting viruses. More importantly, modification and application of insect-infecting viruses can be used as effective biologicals for the control of insect pest. The new viruses identified include Wuhan nodavirus (WhNV), a member of family Nodaviridae; Dendrolimus punctatus tetravirus (DpTV), a new member of the genus Omegatetravirus of the family Alphatetravirida; Ectropis obliqua picorna-like virus (EoV), a positive-strand RNA virus causing a lethal granulosis infection in the larvae of the tea looper (Ectropis obliqua), the virus a member of the Flaviridae family.
While we are enjoying ourselves with the civilization of modern societies, the ecology has ever been changing. Human beings encounter more ecology-climate-changing problems, including the zoonotic pathogens. We have to face some unknown pathogenic agents passively. To get ourselves well prepared we also ought to actively hunt for unknown pathogens. Prediction and pre-warning can only be realized by knowing more about the unknown. This is especially true for infectious agents. | What assay played an important role? | {
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1,057 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | What was the death toll in the 1918-1919 Spanish Influenza epidemic? | {
"answer_start": [
157
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"text": [
"50 million deaths worldwide"
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1,058 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | How many people were infected during the 1918 Spanish Influenza epidemic? | {
"answer_start": [
985
],
"text": [
"An estimated one third of the world’s population (or\nz500 million persons) were infected and had clinical-\nly apparent illnesses"
]
} | false |
1,059 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | What was the case fatality rate in the 1918 Spanish Influenza epidemic? | {
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"Case-\nfatality rates were >2.5%, compared to <0.1% in other\ninfluenza pandemics"
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1,060 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Robertson JS, et al. Specification of receptor-binding phenotypes of
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influenza B viruses share a common high binding affinity for 6′-sialyl(N-acetyllactosamine). Virology. 1997;232: 345–50.
29. Matrosovich M, Gambaryan A, Teneberg S, Piskarev VE, Yamnikova
SS, Lvov DK, et al. Avian influenza A viruses differ from human
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33. Grove RD, Hetzel AM. Vital statistics rates in the United States:
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1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
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38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | What was the death toll in the 1918-1919 Spanish Influenza epidemic? | {
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"Total deaths were estimated at\nz50 million (577) and were arguably as high as 100 mil-\nlion "
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1,062 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Are the modern day Influenza viruses related to the 1918 Spanish Influenza virus? | {
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"text": [
" All influenza A pandemics since that time, and\nindeed almost all cases of influenza A worldwide (except-\ning human infections from avian Viruses such as H5N1 and\nH7N7), have been caused by descendants of the 1918\nVirus, including “drifted” H1N1 Viruses and reassorted\nH2N2 and H3N2 Viruses."
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1,063 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
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1,064 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | When was it determined that the 1918 pandemic was caused by the H1N1 Influenza virus? | {
"answer_start": [
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],
"text": [
"That question did not begin to be resolved until the 1930s,\nwhen closely related influenza Viruses (now known to be\nH1N1 Viruses) were isolated, first from pigs and shortly\nthereafter from humans. Seroepidemiologic studies soon\nlinked both of these viruses to the 1918 pandemic"
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1,065 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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16. Reid AH, Janczewski TA, Lourens RM, Elliot AJ, Daniels RS, Berry
CL, et al. 1918 influenza pandemic caused by highly conserved viruses with two receptor-binding variants. Emerg Infect Dis.
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TG. Characterization of the 1918 influenza virus polymerase genes.
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Robertson JS, et al. Specification of receptor-binding phenotypes of
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29. Matrosovich M, Gambaryan A, Teneberg S, Piskarev VE, Yamnikova
SS, Lvov DK, et al. Avian influenza A viruses differ from human
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TM, et al. A single amino acid substitution in the 1918 influenza virus
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DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
33. Grove RD, Hetzel AM. Vital statistics rates in the United States:
1940–1960. Washington: US Government Printing Office, 1968.
34. Linder FE, Grove RD. Vital statistics rates in the United States:
1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
36. Frost WH. The epidemiology of influenza. Public Health Rep.
1919;34:1823–61.
37. Collins SD. Age and sex incidence of influenza and pneumonia morbidity and mortality in the epidemic of 1928-1929 with comparative
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Did the Spanish Influenza or Swine flu or the H1N1 virus disappear in humans for some time? | {
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" descendants of the 1918\nVirus still persists enzootically in pigs. They probably also\ncirculated continuously in humans, undergoing gradual\nantigenic drift and causing annual epidemics, until the\n1950s. With the appearance of a new H2N2 pandemic\nstrain in 1957 (“Asian flu”), the direct H1N1 Viral descen-\ndants 0f the 1918 pandemic strain disappeared from human\ncirculation entirely, although the related lineage persisted\nenzootically in pigs."
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1,066 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | When did the Swine Flu (Spanish Influenza) virus reappear in humans? | {
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1,068 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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1931;46:1909–37.
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39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | What descendant lineages of the swine flu (Spanish Influenza) virus were identified in 2006? | {
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" 2 major descendant lineages of the 1918\nH1N1 Virus, as well as 2 additional reassortant lineages,\npersist naturally: a human epidemic/endemic H1N1 line-\nage, a porcine enzootic H1N1 lineage (so-called classic\nswine flu), and the reassorted human H3N2 Virus lineage,\nwhich like the human H1N1 Virus, has led to a porcine\nH3N2 lineage."
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1,070 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Are the modern descendant influenza viruses as dangerous as the 1918 parent swine flu (Spanish Influenza) H1N1 virus? | {
"answer_start": [
3605
],
"text": [
"None of these Viral descendants, however,\napproaches the pathogenicity of the 1918 parent Virus."
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} | false |
1,072 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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2004;78:9499–511.
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1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
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1919;34:1823–61.
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | How dangerous are the modern H1N1 (swine flu) and the H3N2 (Influenza A) viruses compared to the 1918 H1N1 (swine flu Spanish Influenza) viruses? | {
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"the human H1N1 and H3N2 lin-\neages have both been associated with substantially lower\nrates ofillness and death than the virus of 1918. In fact, cur-\nrent H1N1 death rates are even lower than those for H3N2\nlineage strains (prevalent from 1968 until the present)."
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1,073 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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2004;78:9499–511.
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1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
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1919;34:1823–61.
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Are the descendant H1N1 strains of the 1918 H1N1 swine flu (Spanish Influenza) virus, still prevalent? | {
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"text": [
"H1N1 Viruses descended from the 1918 strain, as well as \nH3N2 Viruses, have now been cocirculating worldwide for\n29 years and show little evidence of imminent extinction."
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1,075 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Is the origin and epidemiology of the 1918 swine flu (Spanish Influenza) known? | {
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"ongoing studies to map Virulence\nfactors are yielding interesting results. The 1918 sequence\ndata, however, leave unanswered questions about the ori-\ngin of the Virus (19) and about the epidemiology of the\npandemic."
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1,082 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | What is the geographical origin of the H1N1 swine flu ? | {
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"Historical and epidemiologic data are inade-\nquate to identify the geographic origin of the Virus (21),\nand recent phylogenetic analysis of the 1918 Viral genome\ndoes not place the Virus in any geographic context"
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1,087 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
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"Confounding definite assignment of a geographic\npoint of origin, the 1918 pandemic spread more or less\nsimultaneously in 3 distinct waves during an z12-month\nperiod in 191871919, in Europe, Asia, and North America\n(the first wave was best described in the United States in\nMarch 1918)"
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1,088 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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2004;78:9499–511.
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1931;46:1909–37.
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39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | What is an unique feature of the 1918 swine flu? | {
"answer_start": [
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"text": [
"the simultaneous (or nearly simultaneous) infection\nof humans and swin"
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1,089 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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CL, et al. 1918 influenza pandemic caused by highly conserved viruses with two receptor-binding variants. Emerg Infect Dis.
2003;9:1249–53.
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TG. Characterization of the 1918 influenza virus polymerase genes.
Nature. 2005;437:889–93.
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genetic origins of the 1918 pandemic influenza virus. Nat Rev
Microbiol. 2004;2:909–14.
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DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
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1940–1960. Washington: US Government Printing Office, 1968.
34. Linder FE, Grove RD. Vital statistics rates in the United States:
1900–1940. Washington: US Government Printing Office, 1943.
35. Simonsen L, Clarke MJ, Schonberger LB, Arden NH, Cox NJ,
Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
36. Frost WH. The epidemiology of influenza. Public Health Rep.
1919;34:1823–61.
37. Collins SD. Age and sex incidence of influenza and pneumonia morbidity and mortality in the epidemic of 1928-1929 with comparative
data for the epidemic of 1918–1919. Public Health Rep.
1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | What season or time of the year do the new strains of influenza emerge? | {
"answer_start": [
9120
],
"text": [
"Historical records since the 16th century suggest that\nnew influenza pandemics may appear at any time of year,\nnot necessarily in the familiar annual winter patterns of\ninterpandemic years,"
]
} | false |
1,090 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Once appeared, when do the influenza like diseases occur in subsequent years? | {
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1,091 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | When did the first wave of the H1N1 swine flu (Spanish Influenza) occur? | {
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"text": [
" a first or spring wave\nbegan in March 1918 and spread unevenly through the\nUnited States, Europe, and possibly Asia over the next 6\nmonths"
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1,092 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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2004;78:9499–511.
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1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
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1,093 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | When were the second and the third wave of the 1918-1919 swine flu pandemic? | {
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" A sec-\nond or fall wave spread globally from September to\nNovember 1918 and was highly fatal. In many nations, a\nthird wave occurred in early 1919 "
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1,094 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
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are affiliated. | What was the primary difference between the first wave and the 2nd and 3rd wave of the 1918-1919 swine flu pandemic? | {
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10785
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"the much higher fre-\nquency of complicated, severe, and fatal cases in the last 2\nwaves."
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1,096 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Why the human influenza viruses do not disappear after herd immunity is developed? | {
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1,099 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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CL, et al. 1918 influenza pandemic caused by highly conserved viruses with two receptor-binding variants. Emerg Infect Dis.
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TG. Characterization of the 1918 influenza virus polymerase genes.
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TM, et al. A single amino acid substitution in the 1918 influenza virus
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DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
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1940–1960. Washington: US Government Printing Office, 1968.
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1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
36. Frost WH. The epidemiology of influenza. Public Health Rep.
1919;34:1823–61.
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data for the epidemic of 1918–1919. Public Health Rep.
1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | What are the circumstances that promote the spread of influenza virus? | {
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"text": [
" lower environ-\nmental temperatures and human nasal temperatures (bene-\nficial to thermolabile Viruses such as influenza), optimal\nhumidity, increased crowding indoors, and imperfect ven-\ntilation due to closed windows and suboptimal airflow"
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1,101 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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32. Kash JC, Basler CF, Garcia-Sastre A, Carter V, Billharz R, Swayne
DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
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1940–1960. Washington: US Government Printing Office, 1968.
34. Linder FE, Grove RD. Vital statistics rates in the United States:
1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
36. Frost WH. The epidemiology of influenza. Public Health Rep.
1919;34:1823–61.
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data for the epidemic of 1918–1919. Public Health Rep.
1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Do seasonal temperatures and humidity explain the appearance of the three waves of the 1918 swine flu? | {
"answer_start": [
11957
],
"text": [
"such factors cannot explain the 3 pandemic\nwaves of 1918-1919, which occurred in the spring-sum-\nmer, summer—fall, and winter (of the Northern\nHemisphere), respectively. The first 2 waves occurred at a\ntime of year normally unfavorable to influenza Virus\nspread. The second wave caused simultaneous outbreaks\nin the Northern and Southern Hemispheres from\nSeptember to November. "
]
} | false |
1,105 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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data for the epidemic of 1918–1919. Public Health Rep.
1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
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1,106 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
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1940–1960. Washington: US Government Printing Office, 1968.
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1900–1940. Washington: US Government Printing Office, 1943.
35. Simonsen L, Clarke MJ, Schonberger LB, Arden NH, Cox NJ,
Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
36. Frost WH. The epidemiology of influenza. Public Health Rep.
1919;34:1823–61.
37. Collins SD. Age and sex incidence of influenza and pneumonia morbidity and mortality in the epidemic of 1928-1929 with comparative
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Are viruses in the first and third waves of the 1918 swine flu pandemic same or derived from the virus from the second wave of the swine flu? | {
"answer_start": [
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"text": [
"nothing\ncan yet be said about whether the first (spring) wave, or for\nthat matter, the third wave, represented circulation of the\nsame Virus or variants of it"
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1,107 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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32. Kash JC, Basler CF, Garcia-Sastre A, Carter V, Billharz R, Swayne
DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
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1940–1960. Washington: US Government Printing Office, 1968.
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1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
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1919;34:1823–61.
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Was the 1918 swine flu virus novel to humans are was it derived from older viruses? | {
"answer_start": [
14934
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"text": [
"Viral sequence data now suggest that the entire 1918\nVirus was novel to humans in, or shortly before, 1918, and\nthat it thus was not a reassortant Virus produced from old\nexisting strains that acquired 1 or more new genes"
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} | false |
1,108 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
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1940–1960. Washington: US Government Printing Office, 1968.
34. Linder FE, Grove RD. Vital statistics rates in the United States:
1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
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1919;34:1823–61.
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Do avian flu viruses change over long periods? | {
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"text": [
"Influenza Virus gene\nsequences from a number offixed specimens ofwild birds\ncollected circa 1918 show little difference from avian\nViruses isolated today, indicating that avian Viruses likely\nundergo little antigenic change in their natural hosts even\nover long periods"
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1,109 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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1900–1940. Washington: US Government Printing Office, 1943.
35. Simonsen L, Clarke MJ, Schonberger LB, Arden NH, Cox NJ,
Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
36. Frost WH. The epidemiology of influenza. Public Health Rep.
1919;34:1823–61.
37. Collins SD. Age and sex incidence of influenza and pneumonia morbidity and mortality in the epidemic of 1928-1929 with comparative
data for the epidemic of 1918–1919. Public Health Rep.
1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | What is the typical age profile of mortality in Influenza diseases? | {
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"The curve of influenza deaths by age at death has histor-\nically, for at least 150 years, been U-shaped (Figure 2),\nexhibiting mortality peaks in the very young and the very\nold, with a comparatively low frequency of deaths at all\nages in between"
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1,110 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
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1,111 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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2004;78:9499–511.
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1940–1960. Washington: US Government Printing Office, 1968.
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1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
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1919;34:1823–61.
37. Collins SD. Age and sex incidence of influenza and pneumonia morbidity and mortality in the epidemic of 1928-1929 with comparative
data for the epidemic of 1918–1919. Public Health Rep.
1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Which age group was most susceptible to die during the 1918 swine flu pandemic? | {
"answer_start": [
23095
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"text": [
"Persons 65 years of age in 1918 had a dispro-\nportionately high influenza incidence"
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1,112 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
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1,113 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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32. Kash JC, Basler CF, Garcia-Sastre A, Carter V, Billharz R, Swayne
DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
33. Grove RD, Hetzel AM. Vital statistics rates in the United States:
1940–1960. Washington: US Government Printing Office, 1968.
34. Linder FE, Grove RD. Vital statistics rates in the United States:
1900–1940. Washington: US Government Printing Office, 1943.
35. Simonsen L, Clarke MJ, Schonberger LB, Arden NH, Cox NJ,
Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
36. Frost WH. The epidemiology of influenza. Public Health Rep.
1919;34:1823–61.
37. Collins SD. Age and sex incidence of influenza and pneumonia morbidity and mortality in the epidemic of 1928-1929 with comparative
data for the epidemic of 1918–1919. Public Health Rep.
1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
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1,114 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
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1940–1960. Washington: US Government Printing Office, 1968.
34. Linder FE, Grove RD. Vital statistics rates in the United States:
1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
36. Frost WH. The epidemiology of influenza. Public Health Rep.
1919;34:1823–61.
37. Collins SD. Age and sex incidence of influenza and pneumonia morbidity and mortality in the epidemic of 1928-1929 with comparative
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1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Is there a difference in the pathologic feature and course of disease between modern influenza pandemics and the 1918 swine flu pandemic? | {
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"text": [
" the 1918\npandemic was different in degree, but not in kind, from\nprevious and subsequent pandemics. Despite the extraordi-\nnary number of global deaths, most influenza cases in\n1918 (>95% in most locales in industrialized nations) were\nmild and essentially indistinguishable from influenza cases\ntoday. "
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1,117 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
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1940–1960. Washington: US Government Printing Office, 1968.
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1900–1940. Washington: US Government Printing Office, 1943.
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Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
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38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
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Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
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1,120 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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TG. Characterization of the 1918 influenza virus polymerase genes.
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2004;78:9499–511.
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1900–1940. Washington: US Government Printing Office, 1943.
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38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Why was there such a high death rate in the 19118 swine flu pandemic? | {
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1,123 |
1918 Influenza: the Mother of All Pandemics
Jeffery K. Taubenberger" and David M. Morens1-
The “Spanish" influenza pandemic of 1918—1919,
which caused :50 million deaths worldwide, remains an
ominous warning to public health. Many questions about its
origins, its unusual epidemiologic features, and the basis of
its pathogenicity remain unanswered. The public health
implications of the pandemic therefore remain in doubt
even as we now grapple with the feared emergence of a
pandemic caused by H5N1 or other virus. However, new
information about the 1918 virus is emerging, for example,
sequencing of the entire genome from archival autopsy tis-
sues. But, the viral genome alone is unlikely to provide
answers to some critical questions. Understanding the
1918 pandemic and its implications for future pandemics
requires careful experimentation and in-depth historical
analysis.
”Curiouser and curiouser/ ” criedAlice
Lewis Carroll, Alice’s Adventures in Wonderland, 1865
An estimated one third of the world’s population (or
z500 million persons) were infected and had clinical-
ly apparent illnesses (1,2) during the 191871919 influenza
pandemic. The disease was exceptionally severe. Case-
fatality rates were >2.5%, compared to <0.1% in other
influenza pandemics (3,4). Total deaths were estimated at
z50 million (577) and were arguably as high as 100 mil-
lion (7).
The impact of this pandemic was not limited to
191871919. All influenza A pandemics since that time, and
indeed almost all cases of influenza A worldwide (except-
ing human infections from avian Viruses such as H5N1 and
H7N7), have been caused by descendants of the 1918
Virus, including “drifted” H1N1 Viruses and reassorted
H2N2 and H3N2 Viruses. The latter are composed of key
genes from the 1918 Virus, updated by subsequently-incor—
porated avian influenza genes that code for novel surface
*Armed Forces Institute of Pathology, Rockville, Maryland, USA;
and TNational Institutes of Health, Bethesda, Maryland, USA
proteins, making the 1918 Virus indeed the “mother” of all
pandemics.
In 1918, the cause of human influenza and its links to
avian and swine influenza were unknown. Despite clinical
and epidemiologic similarities to influenza pandemics of
1889, 1847, and even earlier, many questioned whether
such an explosively fatal disease could be influenza at all.
That question did not begin to be resolved until the 1930s,
when closely related influenza Viruses (now known to be
H1N1 Viruses) were isolated, first from pigs and shortly
thereafter from humans. Seroepidemiologic studies soon
linked both of these viruses to the 1918 pandemic (8).
Subsequent research indicates that descendants of the 1918
Virus still persists enzootically in pigs. They probably also
circulated continuously in humans, undergoing gradual
antigenic drift and causing annual epidemics, until the
1950s. With the appearance of a new H2N2 pandemic
strain in 1957 (“Asian flu”), the direct H1N1 Viral descen-
dants 0f the 1918 pandemic strain disappeared from human
circulation entirely, although the related lineage persisted
enzootically in pigs. But in 1977, human H1N1 Viruses
suddenly “reemerged” from a laboratory freezer (9). They
continue to circulate endemically and epidemically.
Thus in 2006, 2 major descendant lineages of the 1918
H1N1 Virus, as well as 2 additional reassortant lineages,
persist naturally: a human epidemic/endemic H1N1 line-
age, a porcine enzootic H1N1 lineage (so-called classic
swine flu), and the reassorted human H3N2 Virus lineage,
which like the human H1N1 Virus, has led to a porcine
H3N2 lineage. None of these Viral descendants, however,
approaches the pathogenicity of the 1918 parent Virus.
Apparently, the porcine H1N1 and H3N2 lineages uncom-
monly infect humans, and the human H1N1 and H3N2 lin-
eages have both been associated with substantially lower
rates ofillness and death than the virus of 1918. In fact, cur-
rent H1N1 death rates are even lower than those for H3N2
lineage strains (prevalent from 1968 until the present).
H1N1 Viruses descended from the 1918 strain, as well as
H3N2 Viruses, have now been cocirculating worldwide for
29 years and show little evidence of imminent extinction.
Trying To Understand What Happened
By the early 1990s, 75 years of research had failed to
answer a most basic question about the 1918 pandemic:
why was it so fatal? No Virus from 1918 had been isolated,
but all of its apparent descendants caused substantially
milder human disease. Moreover, examination of mortality
data from the 1920s suggests that within a few years after
1918, influenza epidemics had settled into a pattern of
annual epidemicity associated with strain drifting and sub-
stantially lowered death rates. Did some critical Viral genet-
ic event produce a 1918 Virus of remarkable pathogenicity
and then another critical genetic event occur soon after the
1918 pandemic to produce an attenuated H1N1 Virus?
In 1995, a scientific team identified archival influenza
autopsy materials collected in the autumn of 1918 and
began the slow process of sequencing small Viral RNA
fragments to determine the genomic structure of the
causative influenza Virus (10). These efforts have now
determined the complete genomic sequence of 1 Virus and
partial sequences from 4 others. The primary data from the
above studies (11717) and a number of reviews covering
different aspects of the 1918 pandemic have recently been
published ([8720) and confirm that the 1918 Virus is the
likely ancestor of all 4 of the human and swine H1N1 and
H3N2 lineages, as well as the “extinct” H2N2 lineage. No
known mutations correlated with high pathogenicity in
other human or animal influenza Viruses have been found
in the 1918 genome, but ongoing studies to map Virulence
factors are yielding interesting results. The 1918 sequence
data, however, leave unanswered questions about the ori-
gin of the Virus (19) and about the epidemiology of the
pandemic.
When and Where Did the 1918 Influenza
Pandemic Arise?
Before and after 1918, most influenza pandemics
developed in Asia and spread from there to the rest of the
world. Confounding definite assignment of a geographic
point of origin, the 1918 pandemic spread more or less
simultaneously in 3 distinct waves during an z12-month
period in 191871919, in Europe, Asia, and North America
(the first wave was best described in the United States in
March 1918). Historical and epidemiologic data are inade-
quate to identify the geographic origin of the Virus (21),
and recent phylogenetic analysis of the 1918 Viral genome
does not place the Virus in any geographic context ([9).
Although in 1918 influenza was not a nationally
reportable disease and diagnostic criteria for influenza and
pneumonia were vague, death rates from influenza and
pneumonia in the United States had risen sharply in 1915
and 1916 because of a major respiratory disease epidemic
beginning in December 1915 (22). Death rates then dipped
slightly in 1917. The first pandemic influenza wave
appeared in the spring of 1918, followed in rapid succes-
sion by much more fatal second and third waves in the fall
and winter of 191871919, respectively (Figure 1). Is it pos-
sible that a poorly-adapted H1N1 Virus was already begin-
ning to spread in 1915, causing some serious illnesses but
not yet sufficiently fit to initiate a pandemic? Data consis-
tent with this possibility were reported at the time from
European military camps (23), but a counter argument is
that if a strain with a new hemagglutinin (HA) was caus-
ing enough illness to affect the US national death rates
from pneumonia and influenza, it should have caused a
pandemic sooner, and when it eventually did, in 1918,
many people should have been immune or at least partial-
ly immunoprotected. “Herald” events in 1915, 1916, and
possibly even in early 1918, if they occurred, would be dif-
ficult to identify.
The 1918 influenza pandemic had another unique fea-
ture, the simultaneous (or nearly simultaneous) infection
of humans and swine. The Virus of the 1918 pandemic like-
ly expressed an antigenically novel subtype to which most
humans and swine were immunologically naive in 1918
(12,20). Recently published sequence and phylogenetic
analyses suggest that the genes encoding the HA and neu-
raminidase (NA) surface proteins of the 1918 Virus were
derived from an avianlike influenza Virus shortly before
the start of the pandemic and that the precursor Virus had
not circulated widely in humans or swine in the few
decades before (12,15, 24). More recent analyses of the
other gene segments of the Virus also support this conclu-
sion. Regression analyses of human and swine influenza
sequences obtained from 1930 to the present place the ini-
tial circulation of the 1918 precursor Virus in humans at
approximately 191571918 (20). Thus, the precursor was
probably not circulating widely in humans until shortly
before 1918, nor did it appear to have jumped directly
from any species of bird studied to date (19). In summary,
its origin remains puzzling.
Were the 3 Waves in 1918—1 919 Caused
by the Same Virus? If So, How and Why?
Historical records since the 16th century suggest that
new influenza pandemics may appear at any time of year,
not necessarily in the familiar annual winter patterns of
interpandemic years, presumably because newly shifted
influenza Viruses behave differently when they find a uni-
versal or highly susceptible human population. Thereafter,
confronted by the selection pressures of population immu-
nity, these pandemic Viruses begin to drift genetically and
eventually settle into a pattern of annual epidemic recur-
rences caused by the drifted Virus variants.
Figure 1. Three pandemic waves: weekly combined influenza and
pneumonia mortality, United Kingdom, 1918—1919 (21).
In the 1918-1919 pandemic, a first or spring wave
began in March 1918 and spread unevenly through the
United States, Europe, and possibly Asia over the next 6
months (Figure 1). Illness rates were high, but death rates
in most locales were not appreciably above normal. A sec-
ond or fall wave spread globally from September to
November 1918 and was highly fatal. In many nations, a
third wave occurred in early 1919 (21). Clinical similari-
ties led contemporary observers to conclude initially that
they were observing the same disease in the successive
waves. The milder forms of illness in all 3 waves were
identical and typical of influenza seen in the 1889 pandem-
ic and in prior interpandemic years. In retrospect, even the
rapid progressions from uncomplicated influenza infec-
tions to fatal pneumonia, a hallmark of the 191871919 fall
and winter waves, had been noted in the relatively few
severe spring wave cases. The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.
But 3 extensive pandemic waves of influenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented.
The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human influenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufficiently
limited. The timing and spacing of influenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ficial to thermolabile Viruses such as influenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airflow.
However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The first 2 waves occurred at a
time of year normally unfavorable to influenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the first and second waves with the sharp rises in
cases of the second and third waves is difficult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufficient to produce new influenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world.
At the beginning of other “off season” influenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the first pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months.
What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear.
Because the only 1918 pandemic Virus samples we have
yet identified are from second-wave patients ([6), nothing
can yet be said about whether the first (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from influenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the first wave are inconclusive and do lit-
tle to resolve the question of whether the first wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only influenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question.
What Was the Animal Host
Origin of the Pandemic Virus?
Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike influenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian influenza genes. Influenza Virus gene
sequences from a number offixed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25).
For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reflect evolutionary distance.
Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian influenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
difierences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of influenza
Virus that has not yet been identified or sampled. All of
these findings beg the question: where did the 1918 Virus
come from?
In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain.
Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human influenza strains circulating before
1918 and samples of influenza strains in the wild that more
closely resemble the 1918 sequences.
What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity?
Sequence analysis alone does not ofier clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics.
Influenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site configuration is different for those
influenza Viruses adapted to infect birds and those adapted
to infect humans. Influenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted influenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor configuration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16).
This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—affinity binding to
the human receptor and 1 with mixed-affinity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the first and
third pandemic waves, are unknown.
In a series of in Vivo experiments, recombinant influen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in
mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These findings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inflammatory infiltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown.
Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned.
These experiments may help define the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether specific host factors in 1918 accounted for
unique influenza mortality patterns.
Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults?
The curve of influenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-specific death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Influenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the influenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among influenza pandemics in that absolute risk of
influenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess influenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess influenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33).
A sharper perspective emerges when 1918 age-specific
influenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high influenza incidence (Figure 3, panel A).
But even after adjusting age-specific deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is significantly different from U-shaped age-specific case-
fatality curves typically seen in other influenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
influenza cases, but had a much lower death rate from
influenza and pneumonia than other age groups. To explain
this pattern, we must look beyond properties of the Virus to
host and environmental factors, possibly including
immunopathology (e.g., antibody-dependent infection
enhancement associated with prior Virus exposures [38])
and exposure to risk cofactors such as coinfecting agents,
medications, and environmental agents.
One theory that may partially explain these findings is
that the 1918 Virus had an intrinsically high Virulence, tem-
pered only in those patients who had been born before
1889, e.g., because of exposure to a then-circulating Virus
capable of providing partial immunoprotection against the
1918 Virus strain only in persons old enough (>35 years) to
have been infected during that prior era (35). But this the-
ory would present an additional paradox: an obscure pre-
cursor Virus that left no detectable trace today would have
had to have appeared and disappeared before 1889 and
then reappeared more than 3 decades later.
Epidemiologic data on rates of clinical influenza by
age, collected between 1900 and 1918, provide good evi-
dence for the emergence of an antigenically novel influen-
za Virus in 1918 (21). Jordan showed that from 1900 to
1917, the 5- to 15-year age group accounted for 11% of
total influenza cases, while the >65-year age group
accounted for 6 % of influenza cases. But in 1918, cases in
Figure 2. “U-” and “W—” shaped combined influenza and pneumo-
nia mortality, by age at death, per 100,000 persons in each age
group, United States, 1911—1918. Influenza- and pneumonia-
specific death rates are plotted for the interpandemic years
1911—1917 (dashed line) and for the pandemic year 1918 (solid
line) (33,34).
Incidence male per 1 .nao persunslage group
Mortality per 1.000 persunslige group
+ Case—fataiity rale 1918—1919
Case fatalily par 100 persons ill wilh P&I pel age group
Figure 3. Influenza plus pneumonia (P&l) (combined) age-specific
incidence rates per 1,000 persons per age group (panel A), death
rates per 1,000 persons, ill and well combined (panel B), and
case-fatality rates (panel C, solid line), US Public Health Service
house-to-house surveys, 8 states, 1918 (36). A more typical curve
of age-specific influenza case-fatality (panel C, dotted line) is
taken from US Public Health Service surveys during 1928—1929
(37).
the 5 to 15-year-old group jumped to 25% of influenza
cases (compatible with exposure to an antigenically novel
Virus strain), while the >65-year age group only accounted
for 0.6% of the influenza cases, findings consistent with
previously acquired protective immunity caused by an
identical or closely related Viral protein to which older per-
sons had once been exposed. Mortality data are in accord.
In 1918, persons >75 years had lower influenza and
pneumonia case-fatality rates than they had during the
prepandemic period of 191171917. At the other end of the
age spectrum (Figure 2), a high proportion of deaths in
infancy and early childhood in 1918 mimics the age pat-
tern, if not the mortality rate, of other influenza pandemics.
Could a 1918-like Pandemic Appear Again?
If So, What Could We Do About It?
In its disease course and pathologic features, the 1918
pandemic was different in degree, but not in kind, from
previous and subsequent pandemics. Despite the extraordi-
nary number of global deaths, most influenza cases in
1918 (>95% in most locales in industrialized nations) were
mild and essentially indistinguishable from influenza cases
today. Furthermore, laboratory experiments with recombi-
nant influenza Viruses containing genes from the 1918
Virus suggest that the 1918 and 1918-like Viruses would be
as sensitive as other typical Virus strains to the Food and
Drug Administrationiapproved antiinfluenza drugs riman-
tadine and oseltamivir.
However, some characteristics of the 1918 pandemic
appear unique: most notably, death rates were 5 7 20 times
higher than expected. Clinically and pathologically, these
high death rates appear to be the result of several factors,
including a higher proportion of severe and complicated
infections of the respiratory tract, rather than involvement
of organ systems outside the normal range of the influenza
Virus. Also, the deaths were concentrated in an unusually
young age group. Finally, in 1918, 3 separate recurrences
of influenza followed each other with unusual rapidity,
resulting in 3 explosive pandemic waves within a year’s
time (Figure 1). Each of these unique characteristics may
reflect genetic features of the 1918 Virus, but understand-
ing them will also require examination of host and envi-
ronmental factors.
Until we can ascertain which of these factors gave rise
to the mortality patterns observed and learn more about the
formation of the pandemic, predictions are only educated
guesses. We can only conclude that since it happened once,
analogous conditions could lead to an equally devastating
pandemic.
Like the 1918 Virus, H5N1 is an avian Virus (39),
though a distantly related one. The evolutionary path that
led to pandemic emergence in 1918 is entirely unknown,
but it appears to be different in many respects from the cur-
rent situation with H5N1. There are no historical data,
either in 1918 or in any other pandemic, for establishing
that a pandemic “precursor” Virus caused a highly patho-
genic outbreak in domestic poultry, and no highly patho-
genic avian influenza (HPAI) Virus, including H5N1 and a
number of others, has ever been known to cause a major
human epidemic, let alone a pandemic. While data bearing
on influenza Virus human cell adaptation (e.g., receptor
binding) are beginning to be understood at the molecular
level, the basis for Viral adaptation to efficient human-to-
human spread, the chief prerequisite for pandemic emer-
gence, is unknown for any influenza Virus. The 1918 Virus
acquired this trait, but we do not know how, and we cur-
rently have no way of knowing whether H5N1 Viruses are
now in a parallel process of acquiring human-to-human
transmissibility. Despite an explosion of data on the 1918
Virus during the past decade, we are not much closer to
understanding pandemic emergence in 2006 than we were
in understanding the risk of H1N1 “swine flu” emergence
in 1976.
Even with modern antiviral and antibacterial drugs,
vaccines, and prevention knowledge, the return of a pan-
demic Virus equivalent in pathogenicity to the Virus of
1918 would likely kill >100 million people worldwide. A
pandemic Virus with the (alleged) pathogenic potential of
some recent H5N1 outbreaks could cause substantially
more deaths.
Whether because of Viral, host or environmental fac-
tors, the 1918 Virus causing the first or ‘spring’ wave was
not associated with the exceptional pathogenicity of the
second (fall) and third (winter) waves. Identification of an
influenza RNA-positive case from the first wave could
point to a genetic basis for Virulence by allowing differ-
ences in Viral sequences to be highlighted. Identification of
pre-1918 human influenza RNA samples would help us
understand the timing of emergence of the 1918 Virus.
Surveillance and genomic sequencing of large numbers of
animal influenza Viruses will help us understand the genet-
ic basis of host adaptation and the extent of the natural
reservoir of influenza Viruses. Understanding influenza
pandemics in general requires understanding the 1918 pan-
demic in all its historical, epidemiologic, and biologic
aspects.
Dr Taubenberger is chair of the Department of Molecular
Pathology at the Armed Forces Institute of Pathology, Rockville,
Maryland. His research interests include the molecular patho-
physiology and evolution of influenza Viruses.
Dr Morens is an epidemiologist with a long-standing inter-
est in emerging infectious diseases, Virology, tropical medicine,
and medical history. Since 1999, he has worked at the National
Institute of Allergy and Infectious Diseases.
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influenza B viruses share a common high binding affinity for 6′-sialyl(N-acetyllactosamine). Virology. 1997;232: 345–50.
29. Matrosovich M, Gambaryan A, Teneberg S, Piskarev VE, Yamnikova
SS, Lvov DK, et al. Avian influenza A viruses differ from human
viruses by recognition of sialyloigosaccharides and gangliosides and
by a higher conservation of the HA receptor-binding site. Virology.
1997;233:224–34.
30. Glaser L, Stevens J, Zamarin D, Wilson IA, Garcia-Sastre A, Tumpey
TM, et al. A single amino acid substitution in the 1918 influenza virus
hemagglutinin changes the receptor binding specificity. J Virol.
2005;79:11533–6.
31. Kobasa D, Takada A, Shinya K, Hatta M, Halfmann P, Theriault S, et
al. Enhanced virulence of influenza A viruses with the haemagglutinin of the 1918 pandemic virus. Nature. 2004;431:703–7.
32. Kash JC, Basler CF, Garcia-Sastre A, Carter V, Billharz R, Swayne
DE, et al. Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus. J Virol.
2004;78:9499–511.
33. Grove RD, Hetzel AM. Vital statistics rates in the United States:
1940–1960. Washington: US Government Printing Office, 1968.
34. Linder FE, Grove RD. Vital statistics rates in the United States:
1900–1940. Washington: US Government Printing Office, 1943.
35. Simonsen L, Clarke MJ, Schonberger LB, Arden NH, Cox NJ,
Fukuda K. Pandemic versus epidemic influenza mortality: a pattern
of changing age distribution. J Infect Dis 1998;178:53–60.
36. Frost WH. The epidemiology of influenza. Public Health Rep.
1919;34:1823–61.
37. Collins SD. Age and sex incidence of influenza and pneumonia morbidity and mortality in the epidemic of 1928-1929 with comparative
data for the epidemic of 1918–1919. Public Health Rep.
1931;46:1909–37.
38. Majde JA. Influenza: Learn from the past. ASM News. 1996;62:514.
39. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, et al.
Re-emergence of fatal human influenza A subtype H5N1 disease.
Lancet. 2004;363:617–9.
Address for correspondence: Jeffery K. Taubenberger, Department of
Molecular Pathology, Armed Forces Institute of Pathology, 1413
Research Blvd, Bldg 101, Rm 1057, Rockville, MD 20850-3125, USA;
fax. 301-295-9507; email: [email protected]
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated. | Is the molecular basis of human adaptation of a virus understood? | {
"answer_start": [
29201
],
"text": [
"While data bearing\non influenza Virus human cell adaptation (e.g., receptor\nbinding) are beginning to be understood at the molecular\nlevel, the basis for Viral adaptation to efficient human-to-\nhuman spread, the chief prerequisite for pandemic emer-\ngence, is unknown for any influenza Virus."
]
} | false |
1,872 | Estimating the Unreported Number of Novel Coronavirus (2019-nCoV) Cases in China in the First Half of January 2020: A Data-Driven Modelling Analysis of the Early Outbreak
https://doi.org/10.3390/jcm9020388
SHA: bf20dda99538a594eafc258553634fd9195104cb
Authors: Zhao, Shi; Musa, Salihu S.; Lin, Qianying; Ran, Jinjun; Yang, Guangpu; Wang, Weiming; Lou, Yijun; Yang, Lin; Gao, Daozhou; He, Daihai; Wang, Maggie H.
Date: 2020
DOI: 10.3390/jcm9020388
License: cc-by
Abstract: Background: In December 2019, an outbreak of respiratory illness caused by a novel coronavirus (2019-nCoV) emerged in Wuhan, China and has swiftly spread to other parts of China and a number of foreign countries. The 2019-nCoV cases might have been under-reported roughly from 1 to 15 January 2020, and thus we estimated the number of unreported cases and the basic reproduction number, R0, of 2019-nCoV. Methods: We modelled the epidemic curve of 2019-nCoV cases, in mainland China from 1 December 2019 to 24 January 2020 through the exponential growth. The number of unreported cases was determined by the maximum likelihood estimation. We used the serial intervals (SI) of infection caused by two other well-known coronaviruses (CoV), Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) CoVs, as approximations of the unknown SI for 2019-nCoV to estimate R0. Results: We confirmed that the initial growth phase followed an exponential growth pattern. The under-reporting was likely to have resulted in 469 (95% CI: 403−540) unreported cases from 1 to 15 January 2020. The reporting rate after 17 January 2020 was likely to have increased 21-fold (95% CI: 18−25) in comparison to the situation from 1 to 17 January 2020 on average. We estimated the R0 of 2019-nCoV at 2.56 (95% CI: 2.49−2.63). Conclusion: The under-reporting was likely to have occurred during the first half of January 2020 and should be considered in future investigation.
Text: A novel coronavirus (2019-nCoV) infected pneumonia infection, which is deadly [1] , was first identified in Wuhan, China in December 2019 [2] . The virus causes a range of symptoms including fever, cough, and shortness of breath [3] . The cumulative number of reported cases slowly increased to cumulative 41 cases by 1 January 2020, and rapidly increased after 16 January 2020. As of 26 January 2020, the still ongoing outbreak had resulted in 2066 (618 of them are in Wuhan) confirmed cases and 56 (45 of them were in Wuhan) deaths in mainland China [4] , and sporadic cases exported from Wuhan were reported in Thailand, Japan, Republic of Korea, Hong Kong, Taiwan, Australia, and the United States, please see the World Health Organization (WHO) news release via https://www.who.int/csr/don/en/ from 14 to 21 January 2020. Using the number of cases exported from Wuhan to other countries, a research group at Imperial College London estimated that there had been 4000 (95%CI: 1000-9700) cases in Wuhan with symptoms onset by 18 January 2020, and the basic reproduction number (R 0 ) was estimated at 2.6 (95%CI: 1.5-3.5) [5] . Leung et al. drew a similar conclusion and estimated the number of cases exported from Wuhan to other major cities in China [6] , and the potentials of travel related risks of disease spreading was also indicated by [7] .
Due to an unknown reason, the cumulative number of cases remained at 41 from 1 to 15 January 2020 according to the official report, i.e., no new case was reported during these 15 days, which appears inconsistent with the following rapid growth of the epidemic curve since 16 January 2020. We suspect that the 2019-nCoV cases were under-reported roughly from 1 to 15 January 2020. In this study, we estimated the number of unreported cases and the basic reproduction number, R 0 , of 2019-nCoV in Wuhan from 1 to 15 January 2020 based on the limited data in the early outbreak.
The time series data of 2019-nCoV cases in mainland China were initially released by the Wuhan Municipal Health Commission from 10 to 20 January 2020 [8] , and later by the National Health Commission of China after 21 January 2020 [9] . The case time series data in December 2019 were obtained from a published study [3] . All cases were laboratory confirmed following the case definition by the national health commission of China [10] . We chose the data up to 24 January 2020 instead of to the present study completion date. Given the lag between timings of case confirmation and news release of new cases, the data of the most recent few days were most likely to be tentative, and thus they were excluded from the analysis to be consistent.
We suspected that there was a number of cases, denoted by ξ, under-reported from 1 to 15 January 2020. The cumulative total number of cases, denoted by C i , of the i-th day since 1 December 2019 is the summation of the cumulative reported, c i , and cumulative unreported cases, Ξ i . We have C i = c i + Ξ i , where c i is observed from the data, and Ξ i is 0 for i before 1 January and ξ for i after 15 January 2020. Following previous studies [11, 12] , we modelled the epidemic curve, i.e., the C i series, as an exponential growing Poisson process. Since the data from 1 to 15 January 2020 appeared constant due to unclear reason(s), we removed these data from the fitting of exponential growth. The ξ and the intrinsic growth rate (γ) of the exponential growth were to be estimated based on the log-likelihood, denoted by , from the Poisson priors. The 95% confidence interval (95% CI) of ξ was estimated by the profile likelihood estimation framework with cutoff threshold determined by a Chi-square quantile [13] , χ 2 pr = 0.95, df = 1 . With γ estimated, the basic reproduction number could be obtained by R 0 = 1/M(−γ) with 100% susceptibility for 2019-nCoV presumed at this early stage. Here, the function M(·) was the Laplace transform, i.e., the moment generating function, of the probability distribution for the serial interval (SI) of the disease [11, 14] , denoted by h(k) and k is the mean SI. Since the transmission chain of 2019-nCoV remained unclear, we adopted the SI information from Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS), which share the similar pathogen as 2019-nCoV [15] [16] [17] . We modelled h(k) as Gamma distributions with mean of 8.0 days and standard deviation (SD) of 3.6 days by averaging the SI mean and SD of SARS, mean of 7.6 days and SD of 3.4 days [18] , and MERS, mean of 8.4 days and SD of 3.8 days [19] .
We were also interested in inferring the patterns of the daily number of cases, denoted by ε i for the i-th day, and thus it is obviously that C i = C i−1 + ε i . A simulation framework was developed for the iterative Poisson process such that E[
denoted the expectation. The simulation was implemented starting from 1 January 2020 with a cumulative number of cases seed of 40, the same as reported on 31 December 2019. We conducted 1000 samples and calculated the median and 95% CI.
The number of 2019-nCoV unreported cases was estimated at 469 (95% CI: 403-540), see Figure 1a , which was significantly larger than 0. This finding implied the occurrence of under-reporting between 1 and 15 January 2020. After accounting for the effect of under-reporting, the R 0 was estimated at 2.56 (95% CI: 2.49-2.63), see Figure 1b , which is consistent with many existing online preprints with range from 2 to 4 [5, [20] [21] [22] . With the R 0 of 2.56 and ξ of 469, the exponential growing framework fitted the cumulative total number of cases (C i ) remarkably well, see Figure 1c iterative Poisson process such that
denoted the expectation. The simulation was implemented starting from 1 January 2020 with a cumulative number of cases seed of 40, the same as reported on 31 December 2019. We conducted 1000 samples and calculated the median and 95% CI.
The number of 2019-nCoV unreported cases was estimated at 469 (95% CI: 403−540), see Figure 1a , which was significantly larger than 0. This finding implied the occurrence of under-reporting between 1 and 15 January 2020. After accounting for the effect of under-reporting, the R0 was estimated at 2.56 (95% CI: 2.49−2.63), see Figure 1b , which is consistent with many existing online preprints with range from 2 to 4 [5, [20] [21] [22] . With the R0 of 2.56 and ξ of 469, the exponential growing framework fitted the cumulative total number of cases (Ci) remarkably well, see Figure 1c , referring to McFadden's pseudo-R-squared of 0.99. show the exponential growth fitting results of the cumulative number of cases (Ci) and the daily number of cases (εi) respectively. In panels (c) and (d), the gold squares are the reported cases, the blue bold curve represents the median of the fitting results, the dashed blue curves are the 95% CI of the fitting results, and the purple shading area represents the time window from 1 to 15 January 2020. In panel (c), the blue dots are the cumulative total, i.e., reported and unreported, number of cases. In panel (d), the grey curves are the 1000 simulation samples.
Our estimation of R0 rely on the SI of 2019-nCoV, which remains unknown as of 26 January 2020. In this work, we employed the SIs of SARS and MERS as approximations to that of 2019-nCoV. The determination of SI requires the knowledge of the chain of disease transmission that needs a sufficient number of patient samples and periods of time for follow-up [23] , and thus this is unlikely to be achieved shortly. However, using SIs of SARS and MERS as approximation could provide an panels (a,b) , the green shading area represents the 95% CI (on the horizontal axis), and the vertical green line represents the maximum likelihood estimate (MLE) of the number of unreported cases. With the MLE of R 0 at 2.56, panels (c,d) show the exponential growth fitting results of the cumulative number of cases (C i ) and the daily number of cases (ε i ) respectively. In panels (c,d), the gold squares are the reported cases, the blue bold curve represents the median of the fitting results, the dashed blue curves are the 95% CI of the fitting results, and the purple shading area represents the time window from 1 to 15 January 2020. In panel (c), the blue dots are the cumulative total, i.e., reported and unreported, number of cases. In panel (d), the grey curves are the 1000 simulation samples.
Our estimation of R 0 rely on the SI of 2019-nCoV, which remains unknown as of 26 January 2020. In this work, we employed the SIs of SARS and MERS as approximations to that of 2019-nCoV.
The determination of SI requires the knowledge of the chain of disease transmission that needs a sufficient number of patient samples and periods of time for follow-up [23] , and thus this is unlikely to be achieved shortly. However, using SIs of SARS and MERS as approximation could provide an insight into the transmission potential of 2019-nCoV at the early outbreak. We note that slightly varying the mean and SD of SI would not affect our main conclusions. The R 0 of 2019-nCoV was estimated at 2.56 (95% CI: 2.49-2.63), and it is generally in line with those of SARS, i.e., 2-5 [19, 24, 25] , and MERS, i.e., 2.7-3.9 [26] .
For the simulated daily number of cases (ε i ), see Figure 1d , we found that ε i matched the observed daily number after 17 January 2020, but was significantly larger than the observations from 1 to 17 January 2020. This finding implied that under-reporting was likely to have occurred in the first half of January 2020. We estimated that the reporting rate after 17 January 2020 increased 21-fold (95% CI: [18] [19] [20] [21] [22] [23] [24] [25] compared to the situation from 1 to 17 January 2020 on average. One of the possible reasons was that the official diagnostic protocol was released by WHO on 17 January 2020 [27] , and the diagnosis and reporting efforts of 2019-nCoV infections probably increased. Thereafter, the daily number of newly reported cases started increasing rapidly after 17 January 2020, see Figure 1d . We conducted additional sensitivity analysis by varying the starting date of the under-reporting time window, e.g., 1 January 2020 in the main results, from 2 December 2019 to 3 January 2020, and we report our estimates largely hold. The exact value of the reporting rate was difficult to determine due to lack of serological surveillance data. The reporting rate can be determined if serological surveillance data are available for a population; we would know who was infected (seropositive) and who was not (seronegative), with high confidence. The reporting rate is the ratio of reported cases over the number of seropositive individuals. It was statistically evident that increasing in reporting was likely, and thus it should be considered in the future investigation of this outbreak.
Previous preprint suggested cumulative cases of 1723 (95% CI: 427-4471) as of 12 January 2020, and 4000 (95% CI: 1000-9700) as of 18 January 2020 based on the aggregated international export cases [5] . Our analysis yielded cumulative cases of 280 (95% CI: 128-613) as of 12 January 2020, and 609 (95% CI: 278-1333) as of 18 January 2020 based on the exponential growing mechanistic in the early outbreak. Although our estimate case number appeared to have a lower mean than those estimated by Imai et al. [5] , they are not statistically different. This study applied a different screening effort to detect the 2019-nCoV cases from that in Imai et al. [5] . Imai et al. assumed the average screening effort at overseas airports that covered travelers arriving from Wuhan. Whereas we assumed a constant screening effort applied in Wuhan at the same point of time, and then a number of cases (i.e., ξ) should have been reported yet failed to be reported in the first half of January 2020 due to all sorts of reasons. It is not surprising that different assumptions yielded different results, and this difference in screening effort also partly explained why the detected cases out of China mainly presented mild symptoms. Thus, it was reasonable that our estimates appeared lower than those estimated by Imai et al. [5] . It must be emphasized that such a gap in the knowledge would be resolved by serological survey study (for a large population to approximate the actual positive rate) or an explicit estimation of the actual reporting rate.
Under-reporting was likely to have occurred and resulted in 469 (95% CI: 403-540) unreported cases from 1 to 15 January 2020. The reporting rate after 17 January 2020 was likely to have increased 21-fold (95% CI: 18-25) compared with the situation from 1 to 17 January 2020 on average, and it should be considered in future investigation. We estimated the R 0 at 2019-nCoV to be 2.56 (95% CI: 2.49-2.63).
Author Contributions: All authors conceived the study, carried out the analysis, discussed the results, drafted the first manuscript. All authors have read and agreed to the published version of the manuscript. | What was the initial growth phase pattern? | {
"answer_start": [
1435
],
"text": [
"exponential growth pattern"
]
} | false |
1,875 | Estimating the Unreported Number of Novel Coronavirus (2019-nCoV) Cases in China in the First Half of January 2020: A Data-Driven Modelling Analysis of the Early Outbreak
https://doi.org/10.3390/jcm9020388
SHA: bf20dda99538a594eafc258553634fd9195104cb
Authors: Zhao, Shi; Musa, Salihu S.; Lin, Qianying; Ran, Jinjun; Yang, Guangpu; Wang, Weiming; Lou, Yijun; Yang, Lin; Gao, Daozhou; He, Daihai; Wang, Maggie H.
Date: 2020
DOI: 10.3390/jcm9020388
License: cc-by
Abstract: Background: In December 2019, an outbreak of respiratory illness caused by a novel coronavirus (2019-nCoV) emerged in Wuhan, China and has swiftly spread to other parts of China and a number of foreign countries. The 2019-nCoV cases might have been under-reported roughly from 1 to 15 January 2020, and thus we estimated the number of unreported cases and the basic reproduction number, R0, of 2019-nCoV. Methods: We modelled the epidemic curve of 2019-nCoV cases, in mainland China from 1 December 2019 to 24 January 2020 through the exponential growth. The number of unreported cases was determined by the maximum likelihood estimation. We used the serial intervals (SI) of infection caused by two other well-known coronaviruses (CoV), Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) CoVs, as approximations of the unknown SI for 2019-nCoV to estimate R0. Results: We confirmed that the initial growth phase followed an exponential growth pattern. The under-reporting was likely to have resulted in 469 (95% CI: 403−540) unreported cases from 1 to 15 January 2020. The reporting rate after 17 January 2020 was likely to have increased 21-fold (95% CI: 18−25) in comparison to the situation from 1 to 17 January 2020 on average. We estimated the R0 of 2019-nCoV at 2.56 (95% CI: 2.49−2.63). Conclusion: The under-reporting was likely to have occurred during the first half of January 2020 and should be considered in future investigation.
Text: A novel coronavirus (2019-nCoV) infected pneumonia infection, which is deadly [1] , was first identified in Wuhan, China in December 2019 [2] . The virus causes a range of symptoms including fever, cough, and shortness of breath [3] . The cumulative number of reported cases slowly increased to cumulative 41 cases by 1 January 2020, and rapidly increased after 16 January 2020. As of 26 January 2020, the still ongoing outbreak had resulted in 2066 (618 of them are in Wuhan) confirmed cases and 56 (45 of them were in Wuhan) deaths in mainland China [4] , and sporadic cases exported from Wuhan were reported in Thailand, Japan, Republic of Korea, Hong Kong, Taiwan, Australia, and the United States, please see the World Health Organization (WHO) news release via https://www.who.int/csr/don/en/ from 14 to 21 January 2020. Using the number of cases exported from Wuhan to other countries, a research group at Imperial College London estimated that there had been 4000 (95%CI: 1000-9700) cases in Wuhan with symptoms onset by 18 January 2020, and the basic reproduction number (R 0 ) was estimated at 2.6 (95%CI: 1.5-3.5) [5] . Leung et al. drew a similar conclusion and estimated the number of cases exported from Wuhan to other major cities in China [6] , and the potentials of travel related risks of disease spreading was also indicated by [7] .
Due to an unknown reason, the cumulative number of cases remained at 41 from 1 to 15 January 2020 according to the official report, i.e., no new case was reported during these 15 days, which appears inconsistent with the following rapid growth of the epidemic curve since 16 January 2020. We suspect that the 2019-nCoV cases were under-reported roughly from 1 to 15 January 2020. In this study, we estimated the number of unreported cases and the basic reproduction number, R 0 , of 2019-nCoV in Wuhan from 1 to 15 January 2020 based on the limited data in the early outbreak.
The time series data of 2019-nCoV cases in mainland China were initially released by the Wuhan Municipal Health Commission from 10 to 20 January 2020 [8] , and later by the National Health Commission of China after 21 January 2020 [9] . The case time series data in December 2019 were obtained from a published study [3] . All cases were laboratory confirmed following the case definition by the national health commission of China [10] . We chose the data up to 24 January 2020 instead of to the present study completion date. Given the lag between timings of case confirmation and news release of new cases, the data of the most recent few days were most likely to be tentative, and thus they were excluded from the analysis to be consistent.
We suspected that there was a number of cases, denoted by ξ, under-reported from 1 to 15 January 2020. The cumulative total number of cases, denoted by C i , of the i-th day since 1 December 2019 is the summation of the cumulative reported, c i , and cumulative unreported cases, Ξ i . We have C i = c i + Ξ i , where c i is observed from the data, and Ξ i is 0 for i before 1 January and ξ for i after 15 January 2020. Following previous studies [11, 12] , we modelled the epidemic curve, i.e., the C i series, as an exponential growing Poisson process. Since the data from 1 to 15 January 2020 appeared constant due to unclear reason(s), we removed these data from the fitting of exponential growth. The ξ and the intrinsic growth rate (γ) of the exponential growth were to be estimated based on the log-likelihood, denoted by , from the Poisson priors. The 95% confidence interval (95% CI) of ξ was estimated by the profile likelihood estimation framework with cutoff threshold determined by a Chi-square quantile [13] , χ 2 pr = 0.95, df = 1 . With γ estimated, the basic reproduction number could be obtained by R 0 = 1/M(−γ) with 100% susceptibility for 2019-nCoV presumed at this early stage. Here, the function M(·) was the Laplace transform, i.e., the moment generating function, of the probability distribution for the serial interval (SI) of the disease [11, 14] , denoted by h(k) and k is the mean SI. Since the transmission chain of 2019-nCoV remained unclear, we adopted the SI information from Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS), which share the similar pathogen as 2019-nCoV [15] [16] [17] . We modelled h(k) as Gamma distributions with mean of 8.0 days and standard deviation (SD) of 3.6 days by averaging the SI mean and SD of SARS, mean of 7.6 days and SD of 3.4 days [18] , and MERS, mean of 8.4 days and SD of 3.8 days [19] .
We were also interested in inferring the patterns of the daily number of cases, denoted by ε i for the i-th day, and thus it is obviously that C i = C i−1 + ε i . A simulation framework was developed for the iterative Poisson process such that E[
denoted the expectation. The simulation was implemented starting from 1 January 2020 with a cumulative number of cases seed of 40, the same as reported on 31 December 2019. We conducted 1000 samples and calculated the median and 95% CI.
The number of 2019-nCoV unreported cases was estimated at 469 (95% CI: 403-540), see Figure 1a , which was significantly larger than 0. This finding implied the occurrence of under-reporting between 1 and 15 January 2020. After accounting for the effect of under-reporting, the R 0 was estimated at 2.56 (95% CI: 2.49-2.63), see Figure 1b , which is consistent with many existing online preprints with range from 2 to 4 [5, [20] [21] [22] . With the R 0 of 2.56 and ξ of 469, the exponential growing framework fitted the cumulative total number of cases (C i ) remarkably well, see Figure 1c iterative Poisson process such that
denoted the expectation. The simulation was implemented starting from 1 January 2020 with a cumulative number of cases seed of 40, the same as reported on 31 December 2019. We conducted 1000 samples and calculated the median and 95% CI.
The number of 2019-nCoV unreported cases was estimated at 469 (95% CI: 403−540), see Figure 1a , which was significantly larger than 0. This finding implied the occurrence of under-reporting between 1 and 15 January 2020. After accounting for the effect of under-reporting, the R0 was estimated at 2.56 (95% CI: 2.49−2.63), see Figure 1b , which is consistent with many existing online preprints with range from 2 to 4 [5, [20] [21] [22] . With the R0 of 2.56 and ξ of 469, the exponential growing framework fitted the cumulative total number of cases (Ci) remarkably well, see Figure 1c , referring to McFadden's pseudo-R-squared of 0.99. show the exponential growth fitting results of the cumulative number of cases (Ci) and the daily number of cases (εi) respectively. In panels (c) and (d), the gold squares are the reported cases, the blue bold curve represents the median of the fitting results, the dashed blue curves are the 95% CI of the fitting results, and the purple shading area represents the time window from 1 to 15 January 2020. In panel (c), the blue dots are the cumulative total, i.e., reported and unreported, number of cases. In panel (d), the grey curves are the 1000 simulation samples.
Our estimation of R0 rely on the SI of 2019-nCoV, which remains unknown as of 26 January 2020. In this work, we employed the SIs of SARS and MERS as approximations to that of 2019-nCoV. The determination of SI requires the knowledge of the chain of disease transmission that needs a sufficient number of patient samples and periods of time for follow-up [23] , and thus this is unlikely to be achieved shortly. However, using SIs of SARS and MERS as approximation could provide an panels (a,b) , the green shading area represents the 95% CI (on the horizontal axis), and the vertical green line represents the maximum likelihood estimate (MLE) of the number of unreported cases. With the MLE of R 0 at 2.56, panels (c,d) show the exponential growth fitting results of the cumulative number of cases (C i ) and the daily number of cases (ε i ) respectively. In panels (c,d), the gold squares are the reported cases, the blue bold curve represents the median of the fitting results, the dashed blue curves are the 95% CI of the fitting results, and the purple shading area represents the time window from 1 to 15 January 2020. In panel (c), the blue dots are the cumulative total, i.e., reported and unreported, number of cases. In panel (d), the grey curves are the 1000 simulation samples.
Our estimation of R 0 rely on the SI of 2019-nCoV, which remains unknown as of 26 January 2020. In this work, we employed the SIs of SARS and MERS as approximations to that of 2019-nCoV.
The determination of SI requires the knowledge of the chain of disease transmission that needs a sufficient number of patient samples and periods of time for follow-up [23] , and thus this is unlikely to be achieved shortly. However, using SIs of SARS and MERS as approximation could provide an insight into the transmission potential of 2019-nCoV at the early outbreak. We note that slightly varying the mean and SD of SI would not affect our main conclusions. The R 0 of 2019-nCoV was estimated at 2.56 (95% CI: 2.49-2.63), and it is generally in line with those of SARS, i.e., 2-5 [19, 24, 25] , and MERS, i.e., 2.7-3.9 [26] .
For the simulated daily number of cases (ε i ), see Figure 1d , we found that ε i matched the observed daily number after 17 January 2020, but was significantly larger than the observations from 1 to 17 January 2020. This finding implied that under-reporting was likely to have occurred in the first half of January 2020. We estimated that the reporting rate after 17 January 2020 increased 21-fold (95% CI: [18] [19] [20] [21] [22] [23] [24] [25] compared to the situation from 1 to 17 January 2020 on average. One of the possible reasons was that the official diagnostic protocol was released by WHO on 17 January 2020 [27] , and the diagnosis and reporting efforts of 2019-nCoV infections probably increased. Thereafter, the daily number of newly reported cases started increasing rapidly after 17 January 2020, see Figure 1d . We conducted additional sensitivity analysis by varying the starting date of the under-reporting time window, e.g., 1 January 2020 in the main results, from 2 December 2019 to 3 January 2020, and we report our estimates largely hold. The exact value of the reporting rate was difficult to determine due to lack of serological surveillance data. The reporting rate can be determined if serological surveillance data are available for a population; we would know who was infected (seropositive) and who was not (seronegative), with high confidence. The reporting rate is the ratio of reported cases over the number of seropositive individuals. It was statistically evident that increasing in reporting was likely, and thus it should be considered in the future investigation of this outbreak.
Previous preprint suggested cumulative cases of 1723 (95% CI: 427-4471) as of 12 January 2020, and 4000 (95% CI: 1000-9700) as of 18 January 2020 based on the aggregated international export cases [5] . Our analysis yielded cumulative cases of 280 (95% CI: 128-613) as of 12 January 2020, and 609 (95% CI: 278-1333) as of 18 January 2020 based on the exponential growing mechanistic in the early outbreak. Although our estimate case number appeared to have a lower mean than those estimated by Imai et al. [5] , they are not statistically different. This study applied a different screening effort to detect the 2019-nCoV cases from that in Imai et al. [5] . Imai et al. assumed the average screening effort at overseas airports that covered travelers arriving from Wuhan. Whereas we assumed a constant screening effort applied in Wuhan at the same point of time, and then a number of cases (i.e., ξ) should have been reported yet failed to be reported in the first half of January 2020 due to all sorts of reasons. It is not surprising that different assumptions yielded different results, and this difference in screening effort also partly explained why the detected cases out of China mainly presented mild symptoms. Thus, it was reasonable that our estimates appeared lower than those estimated by Imai et al. [5] . It must be emphasized that such a gap in the knowledge would be resolved by serological survey study (for a large population to approximate the actual positive rate) or an explicit estimation of the actual reporting rate.
Under-reporting was likely to have occurred and resulted in 469 (95% CI: 403-540) unreported cases from 1 to 15 January 2020. The reporting rate after 17 January 2020 was likely to have increased 21-fold (95% CI: 18-25) compared with the situation from 1 to 17 January 2020 on average, and it should be considered in future investigation. We estimated the R 0 at 2019-nCoV to be 2.56 (95% CI: 2.49-2.63).
Author Contributions: All authors conceived the study, carried out the analysis, discussed the results, drafted the first manuscript. All authors have read and agreed to the published version of the manuscript. | What was the result of under-reporting? | {
"answer_start": [
1514
],
"text": [
"469 (95% CI: 403−540) unreported cases from 1 to 15 Januar"
]
} | false |
1,877 | Estimating the Unreported Number of Novel Coronavirus (2019-nCoV) Cases in China in the First Half of January 2020: A Data-Driven Modelling Analysis of the Early Outbreak
https://doi.org/10.3390/jcm9020388
SHA: bf20dda99538a594eafc258553634fd9195104cb
Authors: Zhao, Shi; Musa, Salihu S.; Lin, Qianying; Ran, Jinjun; Yang, Guangpu; Wang, Weiming; Lou, Yijun; Yang, Lin; Gao, Daozhou; He, Daihai; Wang, Maggie H.
Date: 2020
DOI: 10.3390/jcm9020388
License: cc-by
Abstract: Background: In December 2019, an outbreak of respiratory illness caused by a novel coronavirus (2019-nCoV) emerged in Wuhan, China and has swiftly spread to other parts of China and a number of foreign countries. The 2019-nCoV cases might have been under-reported roughly from 1 to 15 January 2020, and thus we estimated the number of unreported cases and the basic reproduction number, R0, of 2019-nCoV. Methods: We modelled the epidemic curve of 2019-nCoV cases, in mainland China from 1 December 2019 to 24 January 2020 through the exponential growth. The number of unreported cases was determined by the maximum likelihood estimation. We used the serial intervals (SI) of infection caused by two other well-known coronaviruses (CoV), Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) CoVs, as approximations of the unknown SI for 2019-nCoV to estimate R0. Results: We confirmed that the initial growth phase followed an exponential growth pattern. The under-reporting was likely to have resulted in 469 (95% CI: 403−540) unreported cases from 1 to 15 January 2020. The reporting rate after 17 January 2020 was likely to have increased 21-fold (95% CI: 18−25) in comparison to the situation from 1 to 17 January 2020 on average. We estimated the R0 of 2019-nCoV at 2.56 (95% CI: 2.49−2.63). Conclusion: The under-reporting was likely to have occurred during the first half of January 2020 and should be considered in future investigation.
Text: A novel coronavirus (2019-nCoV) infected pneumonia infection, which is deadly [1] , was first identified in Wuhan, China in December 2019 [2] . The virus causes a range of symptoms including fever, cough, and shortness of breath [3] . The cumulative number of reported cases slowly increased to cumulative 41 cases by 1 January 2020, and rapidly increased after 16 January 2020. As of 26 January 2020, the still ongoing outbreak had resulted in 2066 (618 of them are in Wuhan) confirmed cases and 56 (45 of them were in Wuhan) deaths in mainland China [4] , and sporadic cases exported from Wuhan were reported in Thailand, Japan, Republic of Korea, Hong Kong, Taiwan, Australia, and the United States, please see the World Health Organization (WHO) news release via https://www.who.int/csr/don/en/ from 14 to 21 January 2020. Using the number of cases exported from Wuhan to other countries, a research group at Imperial College London estimated that there had been 4000 (95%CI: 1000-9700) cases in Wuhan with symptoms onset by 18 January 2020, and the basic reproduction number (R 0 ) was estimated at 2.6 (95%CI: 1.5-3.5) [5] . Leung et al. drew a similar conclusion and estimated the number of cases exported from Wuhan to other major cities in China [6] , and the potentials of travel related risks of disease spreading was also indicated by [7] .
Due to an unknown reason, the cumulative number of cases remained at 41 from 1 to 15 January 2020 according to the official report, i.e., no new case was reported during these 15 days, which appears inconsistent with the following rapid growth of the epidemic curve since 16 January 2020. We suspect that the 2019-nCoV cases were under-reported roughly from 1 to 15 January 2020. In this study, we estimated the number of unreported cases and the basic reproduction number, R 0 , of 2019-nCoV in Wuhan from 1 to 15 January 2020 based on the limited data in the early outbreak.
The time series data of 2019-nCoV cases in mainland China were initially released by the Wuhan Municipal Health Commission from 10 to 20 January 2020 [8] , and later by the National Health Commission of China after 21 January 2020 [9] . The case time series data in December 2019 were obtained from a published study [3] . All cases were laboratory confirmed following the case definition by the national health commission of China [10] . We chose the data up to 24 January 2020 instead of to the present study completion date. Given the lag between timings of case confirmation and news release of new cases, the data of the most recent few days were most likely to be tentative, and thus they were excluded from the analysis to be consistent.
We suspected that there was a number of cases, denoted by ξ, under-reported from 1 to 15 January 2020. The cumulative total number of cases, denoted by C i , of the i-th day since 1 December 2019 is the summation of the cumulative reported, c i , and cumulative unreported cases, Ξ i . We have C i = c i + Ξ i , where c i is observed from the data, and Ξ i is 0 for i before 1 January and ξ for i after 15 January 2020. Following previous studies [11, 12] , we modelled the epidemic curve, i.e., the C i series, as an exponential growing Poisson process. Since the data from 1 to 15 January 2020 appeared constant due to unclear reason(s), we removed these data from the fitting of exponential growth. The ξ and the intrinsic growth rate (γ) of the exponential growth were to be estimated based on the log-likelihood, denoted by , from the Poisson priors. The 95% confidence interval (95% CI) of ξ was estimated by the profile likelihood estimation framework with cutoff threshold determined by a Chi-square quantile [13] , χ 2 pr = 0.95, df = 1 . With γ estimated, the basic reproduction number could be obtained by R 0 = 1/M(−γ) with 100% susceptibility for 2019-nCoV presumed at this early stage. Here, the function M(·) was the Laplace transform, i.e., the moment generating function, of the probability distribution for the serial interval (SI) of the disease [11, 14] , denoted by h(k) and k is the mean SI. Since the transmission chain of 2019-nCoV remained unclear, we adopted the SI information from Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS), which share the similar pathogen as 2019-nCoV [15] [16] [17] . We modelled h(k) as Gamma distributions with mean of 8.0 days and standard deviation (SD) of 3.6 days by averaging the SI mean and SD of SARS, mean of 7.6 days and SD of 3.4 days [18] , and MERS, mean of 8.4 days and SD of 3.8 days [19] .
We were also interested in inferring the patterns of the daily number of cases, denoted by ε i for the i-th day, and thus it is obviously that C i = C i−1 + ε i . A simulation framework was developed for the iterative Poisson process such that E[
denoted the expectation. The simulation was implemented starting from 1 January 2020 with a cumulative number of cases seed of 40, the same as reported on 31 December 2019. We conducted 1000 samples and calculated the median and 95% CI.
The number of 2019-nCoV unreported cases was estimated at 469 (95% CI: 403-540), see Figure 1a , which was significantly larger than 0. This finding implied the occurrence of under-reporting between 1 and 15 January 2020. After accounting for the effect of under-reporting, the R 0 was estimated at 2.56 (95% CI: 2.49-2.63), see Figure 1b , which is consistent with many existing online preprints with range from 2 to 4 [5, [20] [21] [22] . With the R 0 of 2.56 and ξ of 469, the exponential growing framework fitted the cumulative total number of cases (C i ) remarkably well, see Figure 1c iterative Poisson process such that
denoted the expectation. The simulation was implemented starting from 1 January 2020 with a cumulative number of cases seed of 40, the same as reported on 31 December 2019. We conducted 1000 samples and calculated the median and 95% CI.
The number of 2019-nCoV unreported cases was estimated at 469 (95% CI: 403−540), see Figure 1a , which was significantly larger than 0. This finding implied the occurrence of under-reporting between 1 and 15 January 2020. After accounting for the effect of under-reporting, the R0 was estimated at 2.56 (95% CI: 2.49−2.63), see Figure 1b , which is consistent with many existing online preprints with range from 2 to 4 [5, [20] [21] [22] . With the R0 of 2.56 and ξ of 469, the exponential growing framework fitted the cumulative total number of cases (Ci) remarkably well, see Figure 1c , referring to McFadden's pseudo-R-squared of 0.99. show the exponential growth fitting results of the cumulative number of cases (Ci) and the daily number of cases (εi) respectively. In panels (c) and (d), the gold squares are the reported cases, the blue bold curve represents the median of the fitting results, the dashed blue curves are the 95% CI of the fitting results, and the purple shading area represents the time window from 1 to 15 January 2020. In panel (c), the blue dots are the cumulative total, i.e., reported and unreported, number of cases. In panel (d), the grey curves are the 1000 simulation samples.
Our estimation of R0 rely on the SI of 2019-nCoV, which remains unknown as of 26 January 2020. In this work, we employed the SIs of SARS and MERS as approximations to that of 2019-nCoV. The determination of SI requires the knowledge of the chain of disease transmission that needs a sufficient number of patient samples and periods of time for follow-up [23] , and thus this is unlikely to be achieved shortly. However, using SIs of SARS and MERS as approximation could provide an panels (a,b) , the green shading area represents the 95% CI (on the horizontal axis), and the vertical green line represents the maximum likelihood estimate (MLE) of the number of unreported cases. With the MLE of R 0 at 2.56, panels (c,d) show the exponential growth fitting results of the cumulative number of cases (C i ) and the daily number of cases (ε i ) respectively. In panels (c,d), the gold squares are the reported cases, the blue bold curve represents the median of the fitting results, the dashed blue curves are the 95% CI of the fitting results, and the purple shading area represents the time window from 1 to 15 January 2020. In panel (c), the blue dots are the cumulative total, i.e., reported and unreported, number of cases. In panel (d), the grey curves are the 1000 simulation samples.
Our estimation of R 0 rely on the SI of 2019-nCoV, which remains unknown as of 26 January 2020. In this work, we employed the SIs of SARS and MERS as approximations to that of 2019-nCoV.
The determination of SI requires the knowledge of the chain of disease transmission that needs a sufficient number of patient samples and periods of time for follow-up [23] , and thus this is unlikely to be achieved shortly. However, using SIs of SARS and MERS as approximation could provide an insight into the transmission potential of 2019-nCoV at the early outbreak. We note that slightly varying the mean and SD of SI would not affect our main conclusions. The R 0 of 2019-nCoV was estimated at 2.56 (95% CI: 2.49-2.63), and it is generally in line with those of SARS, i.e., 2-5 [19, 24, 25] , and MERS, i.e., 2.7-3.9 [26] .
For the simulated daily number of cases (ε i ), see Figure 1d , we found that ε i matched the observed daily number after 17 January 2020, but was significantly larger than the observations from 1 to 17 January 2020. This finding implied that under-reporting was likely to have occurred in the first half of January 2020. We estimated that the reporting rate after 17 January 2020 increased 21-fold (95% CI: [18] [19] [20] [21] [22] [23] [24] [25] compared to the situation from 1 to 17 January 2020 on average. One of the possible reasons was that the official diagnostic protocol was released by WHO on 17 January 2020 [27] , and the diagnosis and reporting efforts of 2019-nCoV infections probably increased. Thereafter, the daily number of newly reported cases started increasing rapidly after 17 January 2020, see Figure 1d . We conducted additional sensitivity analysis by varying the starting date of the under-reporting time window, e.g., 1 January 2020 in the main results, from 2 December 2019 to 3 January 2020, and we report our estimates largely hold. The exact value of the reporting rate was difficult to determine due to lack of serological surveillance data. The reporting rate can be determined if serological surveillance data are available for a population; we would know who was infected (seropositive) and who was not (seronegative), with high confidence. The reporting rate is the ratio of reported cases over the number of seropositive individuals. It was statistically evident that increasing in reporting was likely, and thus it should be considered in the future investigation of this outbreak.
Previous preprint suggested cumulative cases of 1723 (95% CI: 427-4471) as of 12 January 2020, and 4000 (95% CI: 1000-9700) as of 18 January 2020 based on the aggregated international export cases [5] . Our analysis yielded cumulative cases of 280 (95% CI: 128-613) as of 12 January 2020, and 609 (95% CI: 278-1333) as of 18 January 2020 based on the exponential growing mechanistic in the early outbreak. Although our estimate case number appeared to have a lower mean than those estimated by Imai et al. [5] , they are not statistically different. This study applied a different screening effort to detect the 2019-nCoV cases from that in Imai et al. [5] . Imai et al. assumed the average screening effort at overseas airports that covered travelers arriving from Wuhan. Whereas we assumed a constant screening effort applied in Wuhan at the same point of time, and then a number of cases (i.e., ξ) should have been reported yet failed to be reported in the first half of January 2020 due to all sorts of reasons. It is not surprising that different assumptions yielded different results, and this difference in screening effort also partly explained why the detected cases out of China mainly presented mild symptoms. Thus, it was reasonable that our estimates appeared lower than those estimated by Imai et al. [5] . It must be emphasized that such a gap in the knowledge would be resolved by serological survey study (for a large population to approximate the actual positive rate) or an explicit estimation of the actual reporting rate.
Under-reporting was likely to have occurred and resulted in 469 (95% CI: 403-540) unreported cases from 1 to 15 January 2020. The reporting rate after 17 January 2020 was likely to have increased 21-fold (95% CI: 18-25) compared with the situation from 1 to 17 January 2020 on average, and it should be considered in future investigation. We estimated the R 0 at 2019-nCoV to be 2.56 (95% CI: 2.49-2.63).
Author Contributions: All authors conceived the study, carried out the analysis, discussed the results, drafted the first manuscript. All authors have read and agreed to the published version of the manuscript. | What is R0? | {
"answer_start": [
836
],
"text": [
"basic reproduction number,"
]
} | false |
1,879 | Estimating the Unreported Number of Novel Coronavirus (2019-nCoV) Cases in China in the First Half of January 2020: A Data-Driven Modelling Analysis of the Early Outbreak
https://doi.org/10.3390/jcm9020388
SHA: bf20dda99538a594eafc258553634fd9195104cb
Authors: Zhao, Shi; Musa, Salihu S.; Lin, Qianying; Ran, Jinjun; Yang, Guangpu; Wang, Weiming; Lou, Yijun; Yang, Lin; Gao, Daozhou; He, Daihai; Wang, Maggie H.
Date: 2020
DOI: 10.3390/jcm9020388
License: cc-by
Abstract: Background: In December 2019, an outbreak of respiratory illness caused by a novel coronavirus (2019-nCoV) emerged in Wuhan, China and has swiftly spread to other parts of China and a number of foreign countries. The 2019-nCoV cases might have been under-reported roughly from 1 to 15 January 2020, and thus we estimated the number of unreported cases and the basic reproduction number, R0, of 2019-nCoV. Methods: We modelled the epidemic curve of 2019-nCoV cases, in mainland China from 1 December 2019 to 24 January 2020 through the exponential growth. The number of unreported cases was determined by the maximum likelihood estimation. We used the serial intervals (SI) of infection caused by two other well-known coronaviruses (CoV), Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) CoVs, as approximations of the unknown SI for 2019-nCoV to estimate R0. Results: We confirmed that the initial growth phase followed an exponential growth pattern. The under-reporting was likely to have resulted in 469 (95% CI: 403−540) unreported cases from 1 to 15 January 2020. The reporting rate after 17 January 2020 was likely to have increased 21-fold (95% CI: 18−25) in comparison to the situation from 1 to 17 January 2020 on average. We estimated the R0 of 2019-nCoV at 2.56 (95% CI: 2.49−2.63). Conclusion: The under-reporting was likely to have occurred during the first half of January 2020 and should be considered in future investigation.
Text: A novel coronavirus (2019-nCoV) infected pneumonia infection, which is deadly [1] , was first identified in Wuhan, China in December 2019 [2] . The virus causes a range of symptoms including fever, cough, and shortness of breath [3] . The cumulative number of reported cases slowly increased to cumulative 41 cases by 1 January 2020, and rapidly increased after 16 January 2020. As of 26 January 2020, the still ongoing outbreak had resulted in 2066 (618 of them are in Wuhan) confirmed cases and 56 (45 of them were in Wuhan) deaths in mainland China [4] , and sporadic cases exported from Wuhan were reported in Thailand, Japan, Republic of Korea, Hong Kong, Taiwan, Australia, and the United States, please see the World Health Organization (WHO) news release via https://www.who.int/csr/don/en/ from 14 to 21 January 2020. Using the number of cases exported from Wuhan to other countries, a research group at Imperial College London estimated that there had been 4000 (95%CI: 1000-9700) cases in Wuhan with symptoms onset by 18 January 2020, and the basic reproduction number (R 0 ) was estimated at 2.6 (95%CI: 1.5-3.5) [5] . Leung et al. drew a similar conclusion and estimated the number of cases exported from Wuhan to other major cities in China [6] , and the potentials of travel related risks of disease spreading was also indicated by [7] .
Due to an unknown reason, the cumulative number of cases remained at 41 from 1 to 15 January 2020 according to the official report, i.e., no new case was reported during these 15 days, which appears inconsistent with the following rapid growth of the epidemic curve since 16 January 2020. We suspect that the 2019-nCoV cases were under-reported roughly from 1 to 15 January 2020. In this study, we estimated the number of unreported cases and the basic reproduction number, R 0 , of 2019-nCoV in Wuhan from 1 to 15 January 2020 based on the limited data in the early outbreak.
The time series data of 2019-nCoV cases in mainland China were initially released by the Wuhan Municipal Health Commission from 10 to 20 January 2020 [8] , and later by the National Health Commission of China after 21 January 2020 [9] . The case time series data in December 2019 were obtained from a published study [3] . All cases were laboratory confirmed following the case definition by the national health commission of China [10] . We chose the data up to 24 January 2020 instead of to the present study completion date. Given the lag between timings of case confirmation and news release of new cases, the data of the most recent few days were most likely to be tentative, and thus they were excluded from the analysis to be consistent.
We suspected that there was a number of cases, denoted by ξ, under-reported from 1 to 15 January 2020. The cumulative total number of cases, denoted by C i , of the i-th day since 1 December 2019 is the summation of the cumulative reported, c i , and cumulative unreported cases, Ξ i . We have C i = c i + Ξ i , where c i is observed from the data, and Ξ i is 0 for i before 1 January and ξ for i after 15 January 2020. Following previous studies [11, 12] , we modelled the epidemic curve, i.e., the C i series, as an exponential growing Poisson process. Since the data from 1 to 15 January 2020 appeared constant due to unclear reason(s), we removed these data from the fitting of exponential growth. The ξ and the intrinsic growth rate (γ) of the exponential growth were to be estimated based on the log-likelihood, denoted by , from the Poisson priors. The 95% confidence interval (95% CI) of ξ was estimated by the profile likelihood estimation framework with cutoff threshold determined by a Chi-square quantile [13] , χ 2 pr = 0.95, df = 1 . With γ estimated, the basic reproduction number could be obtained by R 0 = 1/M(−γ) with 100% susceptibility for 2019-nCoV presumed at this early stage. Here, the function M(·) was the Laplace transform, i.e., the moment generating function, of the probability distribution for the serial interval (SI) of the disease [11, 14] , denoted by h(k) and k is the mean SI. Since the transmission chain of 2019-nCoV remained unclear, we adopted the SI information from Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS), which share the similar pathogen as 2019-nCoV [15] [16] [17] . We modelled h(k) as Gamma distributions with mean of 8.0 days and standard deviation (SD) of 3.6 days by averaging the SI mean and SD of SARS, mean of 7.6 days and SD of 3.4 days [18] , and MERS, mean of 8.4 days and SD of 3.8 days [19] .
We were also interested in inferring the patterns of the daily number of cases, denoted by ε i for the i-th day, and thus it is obviously that C i = C i−1 + ε i . A simulation framework was developed for the iterative Poisson process such that E[
denoted the expectation. The simulation was implemented starting from 1 January 2020 with a cumulative number of cases seed of 40, the same as reported on 31 December 2019. We conducted 1000 samples and calculated the median and 95% CI.
The number of 2019-nCoV unreported cases was estimated at 469 (95% CI: 403-540), see Figure 1a , which was significantly larger than 0. This finding implied the occurrence of under-reporting between 1 and 15 January 2020. After accounting for the effect of under-reporting, the R 0 was estimated at 2.56 (95% CI: 2.49-2.63), see Figure 1b , which is consistent with many existing online preprints with range from 2 to 4 [5, [20] [21] [22] . With the R 0 of 2.56 and ξ of 469, the exponential growing framework fitted the cumulative total number of cases (C i ) remarkably well, see Figure 1c iterative Poisson process such that
denoted the expectation. The simulation was implemented starting from 1 January 2020 with a cumulative number of cases seed of 40, the same as reported on 31 December 2019. We conducted 1000 samples and calculated the median and 95% CI.
The number of 2019-nCoV unreported cases was estimated at 469 (95% CI: 403−540), see Figure 1a , which was significantly larger than 0. This finding implied the occurrence of under-reporting between 1 and 15 January 2020. After accounting for the effect of under-reporting, the R0 was estimated at 2.56 (95% CI: 2.49−2.63), see Figure 1b , which is consistent with many existing online preprints with range from 2 to 4 [5, [20] [21] [22] . With the R0 of 2.56 and ξ of 469, the exponential growing framework fitted the cumulative total number of cases (Ci) remarkably well, see Figure 1c , referring to McFadden's pseudo-R-squared of 0.99. show the exponential growth fitting results of the cumulative number of cases (Ci) and the daily number of cases (εi) respectively. In panels (c) and (d), the gold squares are the reported cases, the blue bold curve represents the median of the fitting results, the dashed blue curves are the 95% CI of the fitting results, and the purple shading area represents the time window from 1 to 15 January 2020. In panel (c), the blue dots are the cumulative total, i.e., reported and unreported, number of cases. In panel (d), the grey curves are the 1000 simulation samples.
Our estimation of R0 rely on the SI of 2019-nCoV, which remains unknown as of 26 January 2020. In this work, we employed the SIs of SARS and MERS as approximations to that of 2019-nCoV. The determination of SI requires the knowledge of the chain of disease transmission that needs a sufficient number of patient samples and periods of time for follow-up [23] , and thus this is unlikely to be achieved shortly. However, using SIs of SARS and MERS as approximation could provide an panels (a,b) , the green shading area represents the 95% CI (on the horizontal axis), and the vertical green line represents the maximum likelihood estimate (MLE) of the number of unreported cases. With the MLE of R 0 at 2.56, panels (c,d) show the exponential growth fitting results of the cumulative number of cases (C i ) and the daily number of cases (ε i ) respectively. In panels (c,d), the gold squares are the reported cases, the blue bold curve represents the median of the fitting results, the dashed blue curves are the 95% CI of the fitting results, and the purple shading area represents the time window from 1 to 15 January 2020. In panel (c), the blue dots are the cumulative total, i.e., reported and unreported, number of cases. In panel (d), the grey curves are the 1000 simulation samples.
Our estimation of R 0 rely on the SI of 2019-nCoV, which remains unknown as of 26 January 2020. In this work, we employed the SIs of SARS and MERS as approximations to that of 2019-nCoV.
The determination of SI requires the knowledge of the chain of disease transmission that needs a sufficient number of patient samples and periods of time for follow-up [23] , and thus this is unlikely to be achieved shortly. However, using SIs of SARS and MERS as approximation could provide an insight into the transmission potential of 2019-nCoV at the early outbreak. We note that slightly varying the mean and SD of SI would not affect our main conclusions. The R 0 of 2019-nCoV was estimated at 2.56 (95% CI: 2.49-2.63), and it is generally in line with those of SARS, i.e., 2-5 [19, 24, 25] , and MERS, i.e., 2.7-3.9 [26] .
For the simulated daily number of cases (ε i ), see Figure 1d , we found that ε i matched the observed daily number after 17 January 2020, but was significantly larger than the observations from 1 to 17 January 2020. This finding implied that under-reporting was likely to have occurred in the first half of January 2020. We estimated that the reporting rate after 17 January 2020 increased 21-fold (95% CI: [18] [19] [20] [21] [22] [23] [24] [25] compared to the situation from 1 to 17 January 2020 on average. One of the possible reasons was that the official diagnostic protocol was released by WHO on 17 January 2020 [27] , and the diagnosis and reporting efforts of 2019-nCoV infections probably increased. Thereafter, the daily number of newly reported cases started increasing rapidly after 17 January 2020, see Figure 1d . We conducted additional sensitivity analysis by varying the starting date of the under-reporting time window, e.g., 1 January 2020 in the main results, from 2 December 2019 to 3 January 2020, and we report our estimates largely hold. The exact value of the reporting rate was difficult to determine due to lack of serological surveillance data. The reporting rate can be determined if serological surveillance data are available for a population; we would know who was infected (seropositive) and who was not (seronegative), with high confidence. The reporting rate is the ratio of reported cases over the number of seropositive individuals. It was statistically evident that increasing in reporting was likely, and thus it should be considered in the future investigation of this outbreak.
Previous preprint suggested cumulative cases of 1723 (95% CI: 427-4471) as of 12 January 2020, and 4000 (95% CI: 1000-9700) as of 18 January 2020 based on the aggregated international export cases [5] . Our analysis yielded cumulative cases of 280 (95% CI: 128-613) as of 12 January 2020, and 609 (95% CI: 278-1333) as of 18 January 2020 based on the exponential growing mechanistic in the early outbreak. Although our estimate case number appeared to have a lower mean than those estimated by Imai et al. [5] , they are not statistically different. This study applied a different screening effort to detect the 2019-nCoV cases from that in Imai et al. [5] . Imai et al. assumed the average screening effort at overseas airports that covered travelers arriving from Wuhan. Whereas we assumed a constant screening effort applied in Wuhan at the same point of time, and then a number of cases (i.e., ξ) should have been reported yet failed to be reported in the first half of January 2020 due to all sorts of reasons. It is not surprising that different assumptions yielded different results, and this difference in screening effort also partly explained why the detected cases out of China mainly presented mild symptoms. Thus, it was reasonable that our estimates appeared lower than those estimated by Imai et al. [5] . It must be emphasized that such a gap in the knowledge would be resolved by serological survey study (for a large population to approximate the actual positive rate) or an explicit estimation of the actual reporting rate.
Under-reporting was likely to have occurred and resulted in 469 (95% CI: 403-540) unreported cases from 1 to 15 January 2020. The reporting rate after 17 January 2020 was likely to have increased 21-fold (95% CI: 18-25) compared with the situation from 1 to 17 January 2020 on average, and it should be considered in future investigation. We estimated the R 0 at 2019-nCoV to be 2.56 (95% CI: 2.49-2.63).
Author Contributions: All authors conceived the study, carried out the analysis, discussed the results, drafted the first manuscript. All authors have read and agreed to the published version of the manuscript. | What is likely increase of the reporting rate after the 17th January 2020? | {
"answer_start": [
1649
],
"text": [
"reased 21-fold (95% CI: 18"
]
} | false |
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