Abstract
BackgroundIn April 2009, the most recent pandemic of influenza A began. We present the first estimates of pandemic mortality based on the newly-released final data on deaths in 2009 and 2010 in the United States.MethodsWe obtained data on influenza and pneumonia deaths from the National Center for Health Statistics (NCHS). Age- and sex-specific death rates, and age-standardized death rates, were calculated. Using negative binomial Serfling-type methods, excess mortality was calculated separately by sex and age groups.ResultsIn many age groups, observed pneumonia and influenza cause-specific mortality rates in October and November 2009 broke month-specific records since 1959 when the current series of detailed US mortality data began. Compared to the typical pattern of seasonal flu deaths, the 2009 pandemic age-specific mortality, as well as influenza-attributable (excess) mortality, skewed much younger. We estimate 2,634 excess pneumonia and influenza deaths in 2009–10; the excess death rate in 2009 was 0.79 per 100,000.ConclusionsPandemic influenza mortality skews younger than seasonal influenza. This can be explained by a protective effect due to antigenic cycling. When older cohorts have been previously exposed to a similar antigen, immune memory results in lower death rates at older ages. Age-targeted vaccination of younger people should be considered in future pandemics.
Highlights
In April 2009, a novel strain of influenza A/H1N1 emerged in Mexico, rapidly spreading to the United States [1] and worldwide
The sparing of adults 65 and older is thought to be due to immunity from previous exposure to antigenically-similar H1N1 strains [10,11,12,13,14,15]
Considering death rates weighted over all ages, recent influenza pandemics are not especially severe
Summary
In April 2009, a novel strain of influenza A/H1N1 emerged in Mexico, rapidly spreading to the United States [1] and worldwide. On 11 June 2009, the World Health Organization declared a pandemic [2], which eventually caused an estimated 284,500 deaths worldwide [3]. The pandemic was declared over on 10 August 2010 [4]. 20% of the US population contracted influenza during the pandemic [5,6]. The pandemic case fatality rate was low, and attack rates were higher among children and young adults [9]. In April 2009, the most recent pandemic of influenza A began. We present the first estimates of pandemic mortality based on the newly-released final data on deaths in 2009 and 2010 in the United States
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