LAUSR

BACKGROUND The effectiveness of repeated vaccination for seasonal influenza remains controversial. Here, we measured antibody responses to the influenza virus (A/H1N1, A/H3N2 and B) in a closed cohort of older participants vaccinated against influenza virus in each of five consecutive years. METHODS One hundred and eleven volunteers aged >61 years were vaccinated subcutaneously with one dose (0.5 ml) of inactivated influenza vaccine as recommended by the World Health Organization from the 2005-2006 season through the 2009-2010 season. Hemagglutination inhibition (HI) antibody titers were determined. RESULTS HI antibody titers against all three virus strains were significantly higher at four weeks after vaccination than at a time point prior to vaccination in each of the five seasons (p < 0.01); HI antibody titers were detected at the original pre-vaccination levels just prior to re-vaccination the following year. Sero-protection and HI antibody titers at four weeks after vaccination were similar against all influenza strains and during most of the five seasons evaluated. Vaccine strain changes were associated with specific immune responses in 9 of 12 (75%) intervals. CONCLUSIONS Taken together, our results suggest that annual vaccination is necessary to maintain humoral immunity for the elderly population. Furthermore, our findings revealed that annual seasonal vaccination was not associated with reduced vaccine effectiveness, and that the reformation of the vaccine resulted in amplified immune responses among those undergoing yearly vaccination in the elderly population.