Influenza Mortality in the United States, 2009 Pandemic: Burden, Timing and Age Distribution

Influenza Mortality in the United States, 2009 Pandemic: Burden, Timing and Age Distribution Ann M. Nguyen 1 , Andrew Noymer 2 * 1 Palomar Health, Escondido, California, United States of America, 2 Department of Population Health and Disease Prevention, University of California Irvine, Irvine, California, United States of America Abstract Background: 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. Methods: We 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. Results: In 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. Conclusions: Pandemic 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. Citation: Nguyen AM, Noymer A (2013) Influenza Mortality in the United States, 2009 Pandemic: Burden, Timing and Age Distribution. PLoS ONE 8(5): e64198. doi:10.1371/journal.pone.0064198 Editor: Benjamin J. Cowling, University of Hong Kong, Hong Kong Received December 2, 2012; Accepted April 9, 2013; Published May 22, 2013 Copyright: s 2013 Nguyen, Noymer. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors have no support or funding to report. Competing Interests: Ann M. Nguyen works for Palomar Health, a public health district in San Diego County, California, USA. Palomar Health is not a commercial entity, and therefore no commercial competing interests exist. None of the data analyzed in the article come from Palomar Health. * E-mail: noymer@uci.edu together [16]. Age-specific mortality burden estimates can aid pandemic planning. Introduction In April 2009, a novel strain of influenza A/H1N1 emerged in Mexico, rapidly spreading to the United States [1] and then 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]. Approximately 20% of the US population contracted influenza during the pandemic [5,6]. In 2009, there were 53,692 pneumonia and influenza deaths, making this combination the eighth leading cause of death [7]; in 2010 there were 50,003 pneumonia and influenza deaths (ninth leading cause) [8]. Overall, the pandemic case fatality rate was low, and attack rates were higher among children and young adults [9]. The sparing of adults 65 and older is thought to be due to immunity from previous exposure to antigenically-similar H1N1 strains [10– In this article, we analyze final mortality data for the United States which were released in August 2012. This is the first analysis of the complete mortality record of influenza for 2009 and 2010. Our age- and sex-specific analysis incorporates data from 1959– 2010, and pneumonia and influenza deaths were analyzed PLOS ONE | www.plosone.org Materials and Methods All the data used in this study are fully in the public domain. We obtained data on number of deaths, by cause, from the mortality detail files of the National Center for Health Statistics (NCHS) [17]. Deaths were stratified by age, sex, month, and underlying cause. We extracted data on influenza and pneumonia deaths from January 1959 to December 2010. As shown in table 1, this period spans four revisions of the International Classification of Diseases (ICD 7–10). To ensure comparability, all data were converted to ICD-10 using the published crossover tables [18–20]. Prior to 1959, no detailed mortality data (i.e., simultaneously disaggregated by age, sex, month, and cause) are available. Final mortality data for 2009 and 2010 were released in 2012. Rates (per 100,000) were calculated using these death counts in the numerator, and exposure data (person-years at risk) from the Human Mortality Database [21] in the denominator. The age-specific exposure data were interpolated to monthly units to match the death counts. Variable days per month, including leap years, were used in the monthly exposure interpolation. May 2013 | Volume 8 | Issue 5 | e64198