Abstract
BACKGROUND: Annual vaccination against seasonal influenza viruses is recommended for school-age children in some countries. There are limited data on the immunogenicity and efficacy of repeated influenza vaccinations. METHODS: In a randomized controlled trial, we administered seasonal trivalent inactivated influenza vaccine (TIV) or placebo to 64 children 6-15 years of age in two consecutive years and explored their humoral antibody responses. RESULTS: Receipt of TIV in the first year was associated with lower antibody titer rises in the second year to seasonal influenza A(H1N1) and A(H3N2) strains for which the vaccine strains remained unchanged. Antibody response to a different influenza B strain in the second year was unaffected by receipt of TIV in the first year. Children who received TIV in both years showed higher antibody titers against pandemic A(H1N1) which was not included in either TIV. CONCLUSIONS: Results from our study suggest that humoral antibody response to TIV may be lower in children receiving repeated vaccination, but receipt of TIV induced seroprotection in most subjects. Our study was underpowered to explore whether differences in immunogenicity translated to differences in vaccine efficacy.