[Dosage of inactivated influenza vaccine for infants]

In Japan, the inoculation dosage of inactivated influenza vaccine for children under 1 year old is 0.1 mL per dose. The dosage is not half as much as that in Europe and the U.S.A. We considered that low efficacy fate of influenza vaccine in children under 1 year old results from its less dosage. So we designed this study to verify this hypothesis.

This study was prospective in design. Subjects were divided into two groups by age: 8 to 11 months old (n = 26) and 12 to 16 months old (n = 22). Infants received 0.1 mL of inactivated influenza vaccine and over 1 year, 0.2 mL. Forty-eight children were inoculated twice at intervals of over 4 weeks. Serum samples were drawn before the first inoculation and 1 month after the second vaccination. Pre- and post-immunization antibody titers were measured. The titers of hemaglutinatinin inhibiting antibodies to the 3 viral strains were assayed. Antibody titers were determined using HAI.

The post-vaccination proportions of children with protective HAI antibody titers were significantly smaller in infants than those in children over 1 year old (A/H1N1; 23% vs. 77%, A/H3N2; 39% vs. 73%, B; 0% vs. 32%). The number of children with >four-fold increased antibodies were significantly smaller in infants than that in 1 year old (A/H1N1; 74% vs. 91%, B; 0% vs. 39%). In the mean antibody titer, there were signficant differences between infants and children over 1 year old (A/H1N1; 19 times vs. 56 times, B; 8 times vs. 14 times).

We consider that significant differences in antibody titers between infants and children over 1 year old were caused by the difference of dosage in influenza vaccines. To obtain protective levels of antibodies by influenza vaccines in infants, they must be inoculated with enough dosage.