ArticlesEstimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: a modelling study
Introduction
Influenza pandemics are typically characterised by higher, but widely varying, number of deaths than are seasonal epidemics.1 The emergence of pandemic influenza A H1N1 in April, 2009, led WHO to request that countries report all laboratory-confirmed deaths associated with it. For the period up to August, 2010, 18 500 deaths associated with laboratory-confirmed 2009 pandemic influenza A H1N1 have been reported.2 This number is likely to be an underestimate because diagnostic specimens are not always obtained from people who die with influenza and the viruses might no longer be detectable by the time of death in some people.
Estimation of the 2009-pandemic-associated mortality presents several challenges. First, data for influenza in many countries are sparse and obtained through virological surveillance without standardised case reporting or population denominators needed to estimate incidence. Second, the level and timing of the circulation of the pandemic virus might vary by region and country.3, 4, 5 Third, the severity of influenza might vary by region and country due to differences in access to and quality of health care, nutritional status, prevalence of underlying chronic disorders, age distribution of the populations, and the use of influenza vaccines and antiviral drugs.6, 7, 8, 9, 10
Influenza-associated mortality is often estimated indirectly, by use of statistical models, as the number of excess deaths during periods of circulation of the virus.11, 12, 13 Influenza-associated deaths that might be missed by direct counts of only laboratory-confirmed deaths are taken into account with the use of these indirect approaches. However, indirect estimation might not be as easy to apply in settings where influenza viruses circulate perennially without a clearly defined season or where data for circulation with other respiratory viruses are not available. Additionally, methods to estimate excess mortality require the availability of vital statistics data that are sparse in some regions of the world and not always available in aggregate until 2–3 years after deaths are reported in countries that do have data. Thus, new methods are needed to produce timely and representative estimates of worldwide influenza-associated mortality.
A global estimate of the mortality associated with the 2009 pandemic influenza A H1N1 is needed to document the effect of the pandemic on the world's population and to help guide allocation and delivery of prevention and treatment measures during future pandemics. So far, most reported estimates of mortality associated with 2009 pandemic influenza A H1N1 are from high-income, temperate countries.14, 15, 16, 17 We developed a new approach to estimate global mortality and the number of years of life lost (YLL) associated with the first year of circulation of the 2009 pandemic influenza A H1N1 virus in each country.
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Estimation of respiratory mortality rate
To calculate the base respiratory mortality rate associated with the 2009 pandemic influenza A H1N1 (unadjusted for differences between regions in risk of death) for each country, we estimated the 12 month cumulative symptomatic attack rate (sAR) with data from high-income, middle-income, and low-income countries and multiplied it by the estimated symptomatic case fatality ratio (sCFR) from select high-income countries. sAR was defined as the percentage of the population who developed a
Results
The total of the median estimates of country-specific respiratory deaths associated with the 2009 pandemic influenza A H1N1 was 201 200 (range calculated by summing the 25th and 75th percentile estimates in each age group in each country), more than ten times the number of global deaths reported to WHO for the period April, 2009, to August, 2010 (table 3). Summation of the fifth and 95th percentile estimates in each age group and country would have resulted in an estimated range of
Discussion
During the first year of circulation of the 2009 pandemic influenza A H1N1 virus in each country, an estimated 105 700–395 600 people died of associated respiratory illness. Addition of cardiovascular deaths associated with 2009 pandemic influenza A H1N1 among people older than 17 years increased the mortality burden to 151 700–575 400 deaths (table 3). Our global estimate was more than 15 times higher than the number of laboratory-confirmed deaths reported to WHO during the first 16 months of
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