Prior vaccinations improve immunogenicity of inactivated influenza vaccine in young children aged 6 months to 3 years

Abstract

In young children, infrequent antigen exposure, which is partly characterized by fewer vaccinations, may be a factor impairing the immunogenicity of inactivated influenza vaccine. Weassessed the effects ofprior vaccinationson age-specific immune responses in Japanese children aged6months to3 years, using data froma cohort study with266childrenwhohadreceived2doses (0.25mL/dose for<3yearsold,0.5mL/dosefor3years old) in the2006/2007 season. Serological measures, primarily seroprotection rates, between previously vaccinated and vaccine-naïve children were compared within 1-year age strata. The seroprotection ratewas defined in 2 ways as the proportion of subjectswho achieved an antibody titer of 1:40 or 1:160. Multivariate logistic regression was also performed to estimate the independent effect of prior vaccination on seroprotection rate. After the first dose, seroprotection rates with the threshold of 1:40 in vaccine-naïve 1-year-olds remained low (28% for AH1, 26% for AH3, 2% for B), similar to those of 0-year-olds. In contrast, seroprotection rates in previously vaccinated 1-year-olds (77% for AH1, 86% for AH3, 18% for B) were significantly higher than those in vaccine-naïve 1-year-olds. These seroprotection rates for AH1 and AH3 were comparable with those in previously vaccinated 2- and 3-year-olds. Although seroprotection rates for B remained low in every age stratum even after the second dose, seroprotection rate in previously vaccinated 1-year-olds (50%) was similar to that in 3-year-olds. After adjustment for age, baseline antibody titer and experience of acute febrile respiratory illness in the preceding season, odds ratios showed a significant independent positive effect of prior vaccination on seroprotection rate for every strain. After the seroprotection threshold was changed from 1:40 to 1:160, the results of the effects of prior vaccinations on immunogenicity were similar or became more evident, which demonstrate the robustness of our findings. Our study found that prior vaccinations improved poor immunogenicity among young children, especially in 1-year-olds.