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

Ann Nguyen, Palomar Health

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 aggregate, 2009 was not an unusual year for pneumonia and influenza mortality. Compared to the typical pattern of seasonal flu deaths, pneumonia and influenza age-specific mortality in 2009, as well as influenza-attributable (excess) mortality, skewed much younger. In many age groups, pneumonia and influenza mortality in October and November 2009 broke month-specific records since 1959 when the current series of detail US mortality data began. Mortality in influenza pandemics skews younger than seasonal influenza. This can be explained in part by a protective effect due to antigenic cycling. If older cohorts have been previously exposed to a similar antigen, immune memory can result in lower death rates at older ages.