Immune response after a single vaccination against 2009 influenza A H1N1 in USA: a preliminary report of two randomised controlled phase 2 trials.

BACKGROUND: Data are needed from large clinical trials of paediatric, adult, and elderly people to find the appropriate antigen dose and vaccination schedule for the 2009 pandemic influenza A H1N1. We therefore report preliminary safety and immunogenicity results after one injection of a licensed monovalent pandemic H1N1 vaccine in the USA.

METHODS: We randomly assigned healthy children (aged 6-35 months and 3-9 years) and adults (18-64 years and >or=65 years) to vaccine containing per dose 7.5 microg (children and adults), 15 microg (children and adults), or 30 microg (adults only) haemagglutinin in two placebo-controlled, observer-masked, multicentre phase 2 studies done in the USA. Participants were allocated with an interactive voice-response system or computer-generated randomisation lists with opaque scratchable patches. Primary outcome was haemagglutination inhibition antibody response 21 days after the first of two planned vaccinations (interim analysis of studies in progress). Analyses were by full-analysis set. The trials are registered with ClinicalTrials.gov as NCT00953524 and NCT00952419.

FINDINGS: 410 of 423 children and 724 of 750 adults given an active vaccine, and 50 of 51 children and 95 of 99 adults given placebo were assessed for immunogenicity on day 21. After active vaccination, 45 of 101 (45%; 95% CI 35-55) to 47 of 94 (50%; 40-61) infants aged 6-35 months, 75 of 109 (69%; 59-77) to 80 of 106 (75%; 66-83) 3-9-year-old children, 134 of 141 (95%; 90-98) to 144 of 144 (100%; 98-100) of 18-64-year-old adults, and 93 of 100 (93%; 86-96) to 93 of 98 (95%; 89-98) elderly adults were seroprotected (proportion with titres >or=1:40). No vaccine-related serious adverse events occurred. Injection-site and systemic reactions were reported by up to about 50% of every age and vaccine group, with no noticeable differences between vaccine and placebo groups.

INTERPRETATION: One dose of vaccine was highly immunogenic in adults, suggesting that it afforded sufficient protection against this pandemic influenza A H1N1 virus. Two doses of vaccine will probably be needed in children younger than 9 years. Safety and reactogenicity of the vaccine were acceptable and similar to those of seasonal vaccine.

FUNDING: Office of the Assistant Secretary for Preparedness and Response, and Biomedical Advanced Research and Development Authority.