Immunogenicity and Reactogenicity of a Monovalent Inactivated 2009 Influenza A Vaccine in Adolescents: With Special Reference to Pre-Existing Antibody

Objective To evaluate the immunogenicity and reactogenicity of a monovalent 2009 pandemic influenza vaccine in Japanese adolescents. Study design A total of 111 junior high school and high school students aged 13 to 18 years participated. Subjects received two doses of a monovalent inactivated unadjuvanted 2009 influenza A vaccine. Immunogenicity of the vaccine was evaluated according to the international criteria. We also asked subjects to report adverse reactions. Results After the first dose of vaccine, the seroprotection rate was 91%(95% CI, 85%-96%), the seroconversion rate was 78%(70%-86%), and the geometric mean titer ratio was 11.9 in all subjects. Antibody titers achieved did not differ significantly after the first and the second doses. With multivariate analysis, an independent negative effect of a prevaccination titer of >= 1: 40 on >= 4 fold antibody increase was indicated. No serious adverse reaction was reported. Conclusion The monovalent pandemic vaccine generally was safe, and a single dose of the vaccine given to adolescents induced sufficient immunity. Pre-existing antibody showed substantial effect on antibody response. The effect of pre-existing titer should be considered when evaluating the immunogenicity of influenza vaccines, especially in studies conducted during pandemic waves. (J Pediatr 2012;160:632-7).