6 July 2012
Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: a modeling study
The true impact of the influenza pandemic of 2009 is only now being fully appreciated. Although fatal laboratory cases were reported to the World Health Organization (WHO) during the course of pandemic, the number reported has always been recognized to be a significant undercounting of the true mortality. As laboratory testing was required for confirmation of reported cases, reporting was largely dependent on surveillance and laboratory capacity of countries. Even countries with ample capacity stopped testing every case as laboratories began to get overwhelmed by the volume of requests. Dawood et al have now produced the first global estimate of mortality that attempts to adjust for the under-reporting in the first year of A(H1N1)pdm09 circulation . The authors used a novel approach that involved calculation of a global symptomatic attack rate (sAR) and symptomatic case-fatality rate (sCFR) based on data from surveillance sites around the world. Data for the sAR came from 13 different countries and sCFR from 5. These estimates were then extrapolated to countries without data using a respiratory mortality multiplier based on the ratio of lower respiratory tract infections mortality in each WHO region. A separate estimate of years of life lost was also calculated. The authors estimated that 201 200 (range 105 700 – 395 600) respiratory deaths associated with A(H1N1)pdm09 occurred globally in the first year of the pandemic. A second calculation to account for additional influenza-related cardiac deaths, increased the estimate to 284 400 (range 151 700 – 575 400) deaths. The authors found that 80% of the respiratory and cardiovascular deaths were in people younger than 65 years. The shift in the age distribution of influenza deaths to younger age groups during the pandemic resulted in substantially more years of life lost than would have occurred if the age distribution of fatal cases had been similar to that of seasonal influenza epidemics.
This study by Dawood et al represents the first credible attempt to estimate the true global mortality related to the 2009 influenza pandemic. These estimates are limited by the quality and amount of data available in the time period immediately following the event and the authors have created a novel method to make use of available data from a limited number of pre-existing surveillance systems. The wide variation in observations of rates of illness and death in the countries supplying data results in a fair degree of uncertainty in the final estimates which may be diminished as additional data become available in the future. The more traditional methods of influenza mortality estimation relies on modeling techniques that compare rates of mortality during times of virus transmission to rates during times when the virus is not circulating. These methods, however, use national death registry data which typically several years before they are available. Any method to produce global estimates will constrained by the very limited data available from areas of the world where respiratory disease is thought to have a disproportionately high impact, especially sub-Saharan Africa, and will require some statistical method to extrapolate estimates from countries with data to those without. The years of life lost estimate by the investigators highlights an important aspect of the 2009 pandemic, the relatively young age of the large majority of patients who died. Most of the influenza-associated deaths that occur during a typical season are elderly patients [2,3]. While the absolute number of deaths that occurred during the pandemic of 2009 may be similar to a typical influenza season, the disproportionate impact on younger adults resulted in a much higher number of years of life lost. This finding is consistent with the age distribution observed in the laboratory-confirmed influenza-related deaths reported during the pandemic period. The assessment of the severity of A(H1N1)pdm09 influenza pandemic is important for public health authorities and policy makers as they begin to revise plans for the next event. Estimates of mortality will be extremely useful when combined with estimates of infection to better understand the overall impact of the pandemic.
1. Dawood FS, Iuliano AD, Reed C, Meltzer MI, Shay DK, Cheng P-Y, et al. Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: a modelling study. The Lancet Infectious Diseases. 2012.
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