Blood samples were processed, stored, and shipped by each institution's local Sample Processing Core to Battelle (Columbus, OH). Hemagglutinin inhibition (HAI) testing was performed in duplicate against the influenza vaccine strains in the 2009-2010 Northern Hemisphere influenza vaccine. Although there is debate about the best correlate of protection for influenza,4 seroprotection was defined as an HAI titer of ≥40 since it is the correlate recognized by the United States Food and Drug Administration.5 Seroconversion was measured at S2 and defined as a four-fold rise in HAI post-influenza vaccination compared to pre-vaccination or ≥40 if S1 was <10. Maintenance of antibody response was measured in participants with an S2 ≥40, and defined as an S3≥40. If duplicate HAI results were discrepant by more than two fold, a third test was performed and the minimum result was recorded.