Immunogenicity of the trivalent inactivated influenza vaccine in young children less than 4 years of age, with a focus on age and baseline antibodies.

In this study, we assessed the effects of the prevaccination titer and age on the immunogenicity of a low dose of influenza vaccine in children less than 4 years of age. A total of 259 children received two vaccine doses (0.1 ml for 0-year-olds and 0.2 ml for children 1 year of age or older) 4 weeks apart during the 2005/2006 season. The hemagglutination inhibition antibody titers were measured before vaccination and 4 weeks after the first and second doses. The geometric mean titer, mean fold rise, seroresponse proportion (≥4-fold rise in titer), and seroprotection proportion (titer ≥1:40) were calculated for the prevaccination titer and age categories. A multivariate logistic regression analysis was performed using the seroresponse and seroprotection proportions as dependent variables and the prevaccination titer and age as explanatory variables. As for the seroresponse against the H1 antigen after the first dose, the adjusted odds ratios of the prevaccination titers (versus <1:10) were 2.2 (95% confidence interval, 0.8 to 5.8) at 1:10 to 1:20 and 0.14 (0.04 to 0.49) at ≥1:40. The corresponding figures for ages were 0.03 (0.01 to 0.07) for the 0-year-olds and 0.17 (0.08 to 0.34) for the 1-year-olds compared with the 2- to 3-year-olds (Ptrend < 0.001). Similar results were also obtained for the H3 and B strains. Significantly elevated odds ratios for seroprotection were observed with greater prevaccination titers and older ages for all strains. The prevaccination titer and age were independently associated with the antibody response in young children. The immune response was weaker in the younger children and those without preexisting immunity.