Safety and immunogenicity of an inactivated split-virus influenza A/H1N1 vaccine in healthy children from 6 months to <18 years of age: a prospective, open-label, multi-center trial.

This study was conducted to determine the immunogenicity and safety of an inactivated split-virus influenza A/H1N1 vaccine in healthy Korean children from 6 months to <18 years of age. The immunization schedule consisted of two vaccinations, 21 days apart. The unadjuvanted vaccine contained 7.5microg (subjects 6 months to <3 years of age) or 15microg (subjects 3 to <18 years of age) of hemagglutinin antigen per dose. A total of 251 subjects were enrolled and 248 and 242 subjects, respectively, were included in the post-first dose and post-second dose immunogenicity evaluations conducted on a per protocol basis. By day 21, after the first dose, hemagglutination-inhibition titers of 1:40 or more were observed in 5.9% of subjects 6 months to <3 years of age, 34.9% of subjects 3 to <9 years of age and 81.4% of subjects 9-18 years of age. By day 21 after the second dose, the titer had been achieved 55.9%, 69.5% and 90.5%, respectively. No vaccination-related serious adverse events were observed. A single 15-microg dose of vaccine was highly immunogenic in subjects equal to or more than 9 years of age. However, a two-dose regimen is needed to produce potentially protective antibody titers in younger children.