Fifty seven hospital workers received influenza vaccine in November 2003, and the serum HI antibody titer was determined before, 2 and 4 weeks after the vaccination. Thirty seven were vaccinated in November, 2002 consecutively (the repeated vaccination group), and the remaining 20 had not been vaccinated in the previous year (the single vaccination group). Six of the repeated vaccination group received both influenza and hepatitis B vaccination in September, 2004 and the antibody responses were examined 2 weeks later. Two and four weeks after the 2003-vaccination, the HI antibody titers to A/ H1N1, A/H3N2, and B in the repeated vaccination group were significantly lower than in the single vaccination group (P < 0.05). This phenomenon had no relation to the pre-vaccination HI antibody titer. The antibody response was low to repeated influenza vaccination, but normal to hepatitis B vaccine in six subjects who had a second vaccination in 2004, showing that this depressed response was influenza-specific. These results suggest that the decreased HI antibody response to repeated influenza vaccination was affected mainly by the previous vaccination per se rather than by the pre-existing antibody titer.
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