Influenza and its complications are related to increased morbidity and mortality in elderly persons. Influenza vaccination is an important strategy to minimize the excess morbidity and mortality caused by influenza infection. Influenza epidemics start in November and continue to March in the next year, but it is not clear if the immunization-induced antibody titer lasts throughout the influenza season. One hundred eighty-two rural community-dwelling elderly persons older than 61 years were immunized with one dose of influenza vaccine in the 2005-2006 influenza season, and the antibody response after vaccination was investigated. Both the antibody titers and seroprotection rates were significantly higher four weeks after the vaccination but decreased markedly at 22 weeks after the vaccination. Analysis of the factors affecting the antibody response showed that those who have a low antibody titer before vaccination ('seronegative') do not acquire enough antibody with one dose of influenza vaccine. Neither age nor vaccination in the previous year was related to the antibody response after vaccination. These findings suggest that future clinical trials should be performed to determine if a second booster vaccination maintains the titer.
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