Immunogenicity of a monovalent 2009 influenza A (H1N1) vaccine in infants: randomized, observer-masked, single-center clinical study

ABSTRACT The aim of this study is to further investigate the immune response of the inactivated split-virion vaccine for infants. We tested the immunogenicity and safety of the inactivated split-virion vaccine in infants, aged 6-35 months, for a randomized, observer-masked, age-stratified clinical study. We randomly divided subjects into three groups: 7.5 μg, 15 μg of hemagglutinin antigen dosage groups and seasonal influenza vaccine for children dosage group in a 2 dose regimen. A serologic analysis was performed at baseline and on day 21 and 42. 312 infants received a single dose injection of vaccine and 265 (84.94%) infants received two doses injection of vaccine. Adverse reactions were mostly mild or moderate. Among the subjects who received 7.5 μg and 15 μg of vaccine for a single dose injection, the rate of hemagglutinin inhibition titer of 1:40 or greater were 52.48% (95% confidence interval (CI) 42.83 ~ 61.95) and 61.11% (95% CI 50.78 ~ 70.53), respectively. Among the subjects receiving 7.5 μg and 15 μg of vaccine for two doses injection, the rate of hemagglutinin inhibition (HI) titer of 1:40 or greater were 90.10% (95% CI 82.73 ~ 94.53) and 94.44% (95% CI 87.64 ~ 97.60), respectively. These data suggests that 15 μg or 7.5 μg dose of hemagglutinin antigen of the inactivated split-virion vaccine was safe and two doses of injection could induce a sufficient protective immune response in infants. TRIAL REGISTRATION CLINICAL TRIALS REGISTRATION NCT01494740.