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In this study, we report the effectiveness of trivalent inactivated influenza vaccination (TIV) for severely multiply handicapped persons/children (SMHPs) in the 2005-2006 season. In 77 SMHPs, A/New York/55/2004 (H3N2) which was the changed vaccine-strain showed significant differences in the geometric mean titers (P0.05) and seroprotection rates (P0.01) between pre- and post-vaccination. A/New Caledonia/20/99 (H1N1) and B/Shanghai/361/2002, which were the unchanged vaccine-strains, showed no significant differences. We defined the potential responders as those who can achieve 1:40 or more hemagglutination inhibition (HAI) titer after vaccination with any vaccine-strain. Therefore, the rate of potential responders is equivalent to the rate of seroprotection, estimated to be 40-60% among the SMHPs and80% among the control group in this study. In the SMHPs, even potential responders could only achieve limited HAI titers (1:40-80) even after repeated vaccination. In contrast, the control group showed higher HAI titers compared to the SMHPs for the unchanged vaccine-strains caused by the priming effect. These data suggest that it might be difficult for SMHPs (including potential responders) to achieve the priming effect by the current TIV. Consequently, they cannot obtain a booster effect.