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Approximately 40 children aged 6–36 months were enrolled in a study to evaluate the immunogenicity of the trivalent influenza sub-unit vaccine in the 1999/2000 and 2000/2001 influenza seasons. In 1999/2000, the vaccine dose was 0.1 ml for children aged 6–11 months and 0.2 ml for those aged 12+ months, according to the formulary in Japan. In 2000/2001, the vaccine dose was 0.25 ml, irrespective of age, according to the recommendations of the Advisory Committee on Immunization Practices (ACIP). All children were given two doses approximately 4 weeks apart each year. In 1999/2000, the geometric mean titers and frequency of achieved, protective antibody titers (≧1:40) of children under 12 months of age to those aged 12+ months was 26.3±4.5 vs. 30.9±2.1 (p=0.582) and 50% vs. 52% (p=0.0954) for AH1N1 and 42.9±6.4 vs. 192.5±4.3 (p=0.029) and 60% vs. 87% (p=0.063) for AH3N2 at 4 weeks after the second vaccinations. In 2000/2001, the geometric mean titers of children under 12 months of age to those aged 12+ months was 17.5±2.9 vs. 126.5±4.2 (p=0.002) for AH1N1 and 20.1±2.0 vs. 66.7±4.5 (p=0.039) for AH3N2, respectively, and the frequency of achieved, protective antibody titers was 29% vs. 78% (p=0.010) and 43% vs. 67% (p=0.239), respectively. Risk factors including age, vaccine dose (1999/2000 season), vaccination of last year and pre-vaccination titer were assessed by univariate and multivariate analysis to evaluate their associations with the frequency of a four-fold or greater titer rise when comparing HI titers for specimens obtained 4 weeks after the second vaccination and before the first vaccination. In 1999/2000, an independent negative effect was observed for age. In 2000/2001, children under 12 months of age had a lower risk than those over 12+ months for a four-fold or greater titer rise, OR 0.4 (CI 0.05–3.04) for AH1N1 and OR 0.6 (CI 0.11–3.88) for AH3N2. The children under 12 months seemed to exhibit a reduced response to the influenza sub-unit vaccine. The observed values were not statistically significant because our study population was small. These results strongly suggest the necessity for influenza vaccine administration for all household contacts and out-of-home caretakers of children under 12 months old. |
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