ArticlesEstimation of potential global pandemic influenza mortality on the basis of vital registry data from the 1918–20 pandemic: a quantitative analysis
Introduction
The avian influenza epidemic in birds and the 258 cases recorded in human beings (as of Nov 29, 2006) in several continents1 are generating tremendous media coverage, public concern, and policy debate.2 Governments and donor agencies have joined together to pledge substantial funds to fight the spread of avian influenza;3 for example, the US government has committed $3·8 billion for the USA4 and Australia has set aside AUD$555 million.5 This high degree of concern is in part due to estimates of potential mortality from a major influenza pandemic. Estimates from 2 million6 to 360 million7 and even up to 1 billion8 deaths have been proposed. These numbers, combined with predictions of the inevitability of the next influenza pandemic, are driving continued attention and policy focus.9
Various models of the effect of influenza pandemics on mortality have been developed.10, 11, 12 These models make strong assumptions about attack rates and case-fatality rates in influenza cases. Irrespective of the modelling assumptions, however, the three pandemics of the 20th century—in 1918–20, 1957–58, and 1968–70—are the main source of empirical evidence on the potential human toll of the next pandemic.13, 14, 15 The 1918–20 Spanish flu pandemic caused the highest mortality by far and is often used to set the upper bound on the number of deaths caused by a future pandemic.16 Medical historians have generated estimates of mortality in 1918–20 ranging from 20 million to 100 million.17, 18, 19 These estimates are based on reviews of various historical documents, including national commissions, eye-witness accounts, and local government reports. With some exceptions,20, 21, 22 these analyses have not distinguished quantitative analyses based on underlying high-quality vital registration data from qualitative accounts.
Systematic analysis of all available vital registration data would permit the calculation of pandemic mortality due to the major 20th century influenza pandemics in a comparable manner. Here, our aim is to assess vital registration data from the 1918–20 pandemic, since this pandemic was overwhelmingly larger than other 20th century pandemics and provides a clearly identifiable effect on mortality. We aim to develop statistical models that relate annual pandemic mortality to community attributes, and to use these models to estimate the effect on mortality of an influenza pandemic in 2004, the most recent year for which per-head gross domestic product in international dollars is available.
Section snippets
Data collection
We included all available vital registration data from 1915 to 1923—ie around the period of the 1918–20 influenza pandemic—from populations in which vital registration is believed to be 80% or more complete.23, 24 Data were mainly taken from the Berkeley Human Mortality Database25 and B R Mitchell's International Historical Statistics Series.26 These sources were supplemented with subnational data for US states27, 28, 29, 30, 31, 32, 33, 34, 35 and what were then described as provinces of
Results
Table 1 shows pandemic mortality calculations for 27 countries for 1918–20, 24 US states with data available for the period, and nine Indian provinces. Pandemic mortality rates for the UK, France, and Finland for 1918–20 are based on females only, since male mortality is confounded by deaths due to war. In this sample of countries that are heavily weighted to higher income, median pandemic excess mortality was 0·75 deaths per 100 people (henceforth indicated as %) and average excess mortality
Discussion
Were a strain of influenza much the same as that which caused the 1918–20 pandemic to emerge in 2004, we estimate that it could kill 51–81 million individuals. This estimate is based strictly on recorded patterns of mortality in countries with nearly complete vital registration systems, rather than on theoretical models or assumptions about attack rates and case-fatality rates.
Our results indicate that deaths would be concentrated in the 0–14, 15–19, and 30–44 years age-groups. Various theories
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