Kinetics of the antibody response to seasonal influenza vaccination among the elderly.

Influenza vaccination, which has been targeted to the elderly and those at serious risk of complications, is recommended. The purpose of this study was to determine antibody responses after influenza vaccination among Thai elderly persons living in the community. A total of 591 subjects consisting of 308 vaccinees and 283 non-vaccinees were enrolled in the study. Antibodies to H1N1, H3N2, and B viruses were detected by hemagglutination inhibition (HI) testing. The numbers of subjects who had protective antibody titers ≥40 and geometric mean titers (GMTs) of antibodies against A(H1N1), A(H3N2), and B viruses prior to vaccination were similar for the vaccine and placebo groups. The seroprotection rates and GMTs for influenza virus A(H1N1), A(H3N2), and B strains after influenza vaccination at 1, 5, and 12 mo in the vaccine group were significantly higher than those in the placebo group. The seroprotection rates for the A(H1N1) and A(H3N2) strains, but not the B strain, met Committee for Proprietary Medicinal Products (CPMP) criteria (>60%). GMTs and seroprotection rates against influenza B strain in the vaccinees at all time points were <40% and <60%, respectively, and significant differences between the vaccinees and the placebo controls were observed. The GMTs and seroprotection rates for influenza strains in those with pre-existing antibody titers ≥40 were significantly higher than those in the group with pre-existing antibody titers <40. These findings demonstrated that the elderly living in the community developed adequate antibody responses with sustainable titers throughout the 12-month study period after influenza vaccine immunization. Moreover, the presence of pre-existing antibody at a titer ≥40 prior to vaccination strongly affected the antibody response to influenza vaccination.