Hsu PS, et al. - Researchers undertook this historical cohort investigation involving persons older than 64 years, to assess how repeated vaccination can influence vaccine effectiveness (VE) for influenza-associated hospitalization among elderly in Taiwan, during years with and without early appearance of antigenically drifted strains. Two influenza seasons were covered: 2007– 08 and 2008– 09. A significant decline in hospitalization risk was evident in relation to only current-year vaccination in combination with the previous history of annual revaccination, with adjusted hazard ratios estimated to be 0.68 and 0.74 during the 2007– 08 and 2008– 09 influenza seasons, respectively. On further stratification, it was revealed that influenza-associated hospitalization was significantly decreased by sequential vaccinations, in the female population aged 68– 74 and 75– 84 years, even during the 2007– 08 influenza season when all vaccinations were mismatched with the circulating strains, with adjusted VE of 25.2% and 36.9%, respectively. The findings lend support to the advice of annual revaccination against influenza among the elderly, even though the circulating strain of influenza virus was antigenically mismatched with the vaccine strains.