Preliminary Estimates of Mortality and Years of Life Lost Associated with the 2009 A/H1N1 Pandemic in the US and Comparison with Past Influenza Seasons

Cecile Viboud,Mark Miller,Lone Simonsen,Donald R Olson,Michael Osterholm

doi:10.1371/currents.rrn1153

Abstract

The on-going debate about the health burden of the 2009 influenza pandemic and discussions about the usefulness of vaccine recommendations has been hampered by an absence of directly comparable measures of mortality impact. Here we set out to generate an "apples-to-apples" metric to compare pandemic and epidemic mortality. We estimated the mortality burden of the pandemic in the US using a methodology similar to that used to generate excess mortality burden for inter-pandemic influenza seasons. We also took into account the particularly young age distribution of deaths in the 2009 H1N1 pandemic, using the metric "Years of Life Lost" instead of numbers of deaths. Estimates are based on the timely pneumonia and influenza mortality surveillance data from 122 US cities, and the age distribution of laboratory-confirmed pandemic deaths, which has a mean of 37 years. We estimated that between 7,500 and 44,100 deaths are attributable to the A/H1N1 pandemic virus in the US during May-December 2009, and that between 334,000 and 1,973,000 years of life were lost. The range of years of life lost estimates includes in its lower part the impact of a typical influenza epidemic dominated by the more virulent A/H3N2 subtype, and the impact of the 1968 pandemic in its upper bound. We conclude that the 2009 A/H1N1 pandemic virus had a substantial health burden in the US over the first few months of circulation in terms of years of life lost, justifying the efforts to protect the population with vaccination programs. Analysis of historic records from three other pandemics over the last century suggests that the emerging pandemic virus will continue to circulate and cause excess mortality in unusually young populations for the next few years. Continuing surveillance for indicators of increased mortality is of key importance, as pandemics do not always cause the majority of associated deaths in the first season of circulation.

Highlights

The World Health Organization (WHO) has been criticized for responding too forcefully to the 2009 A/H1N1pdm pandemic threat
Based on the study reported in August 2009 in [9], more than 85% of laboratoryconfirmed A/H1N1 deaths occurred in people under 60 years of age, with a mean age of deaths of 37 years (Table 1). This is in marked contrast to seasonal influenza epidemics where 90% of deaths occur in people over 65 years and the mean age of influenza-related deaths is estimated at 76 yrs
The excess mortality estimates based on the final vital statistics compiled by National Center for Health Statistics (NCHS) and the preliminary data provided by the 122 cities surveillance showed strong agreement for the period 1999-2006, despite the low number of seasons available for comparison (R2>0.80 for pneumonia and influenza (P&I) and all cause excess mortality, Figure 2)

Summary

Introduction

The World Health Organization (WHO) has been criticized for responding too forcefully to the 2009 A/H1N1pdm pandemic threat. The WHO global estimate of ~16,000 laboratory-confirmed deaths from A/H1N1pdm influenza as of February 26, 2010 [2] appears many fewer than the millions of deaths associated with the three previous pandemics of 1918, 1957 and 1968 [3],[4],[5], or even than deaths associated with seasonal influenza epidemics [6],[7]. These figures are not comparable for several reasons. These ~12,000 deaths are qualitatively different than the ~36,000 seasonal influenza deaths which occur in an average winter in the US [6]-- the majority of pandemic deaths have occurred in children and younger adults [9],[10], while almost all seasonal influenza deaths occur in the elderly [7]

Methods

Results

Conclusion