Dosage of Inactivated Influenza Vaccine for Infants

Abstract

Background: In Japan, the inoculation dosage of inactivated influenza vaccine for hildren under 1 year old is 0.1mL per dose. The dosage is not half as much as that in Europe and the U.S.A. Weconsidered that low efficacy fate of influenza vaccine in children under 1 year old results from its lessdosage. So we designed this study to verify this hypothesis.
Materials and Methods: This study was prospective in design. Subjects were dividedinto twogroups by age: 8 to 11 months old (n=26) and 12 to 16 months old (n=22). Infantsreceived 0.1mLof inactivated influenza vaccine and over 1 year, 0.2mL. Forty-eight children wereinoculated twice atintervals of over 4 weeks. Serum samples were drawn before the first inoculation and 1 month afterthe second vaccination. Pre and post immunization antibody titers were measured. The titers ofhemaglutinatinin inhibiting antibodies to the 3 viral strains were assayed. Antibody titers were de-termined using HAI.
Results: The post-vaccination proportions of children with protective HAI antibodytiters weresignificantly 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 increasedantibodieswere 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 hildren over 1year old (A/H1N1; 19 times vs. 56 times, B; 8 times vs. 14 times).
Conclusion: We consider that significant differences in antibody titers between infants and chil-dren over 1 year old were caused by the difference of dosage in influenza vaccines. To obtain protec-tive levels of antibodies by influenza vaccines in infants, they must be inoculated with enough dosage.