AS03(A)-Adjuvanted influenza A (H1N1) 2009 vaccine for adults up to 85 years of age.

Background. Vaccination of high-risk groups was started shortly after the emergence of the influenza A (H1N1) 2009 pandemic virus. Methods. Healthy adults were enrolled into 2 age strata: 18-60 years and >60 years, and received monovalent influenza vaccine containing 3.75 mu g of A/California/2009 (H1N1) hemagglutinin antigen, adjuvanted with AS03(A). Hemagglutination inhibition assay-based antibody titers against H1N1 vaccine were assessed after 1 vaccine dose (primary endpoint), after which subjects were randomized 1:1 to receive no further vaccination or a second dose. Immunogenicity endpoints were European licensure criteria for influenza vaccines. Exploratory analyses assessed the effect of previous seasonal influenza vaccination on responses to the H1N1 vaccine. Results. Licensure criteria for immunogenicity were fulfilled after 1 dose of H1N1 vaccine (N = 240). For subjects 18-60 years of age, previous vaccination against seasonal influenza within the preceding 2 seasons resulted in significantly lower geometric mean titers (adjusted for baseline antibody titer) after 1 or 2 doses of H1N1 vaccine (P < .001 and P = .003, respectively). Transient mild or moderate injection-site pain was reported by 87.5% and 65.0% of subjects 18-60 years of age and >60 years of age, respectively, after the first dose, and in 63% of subjects overall after the second dose. Conclusions. A single dose of 3.75 mu g hemagglutinin antigen, AS03(A)-adjuvanted H1N1 2009 vaccine was immunogenic and well tolerated in adults. In exploratory analyses (of subjects 18-60 years of age), postvaccination antibody titers were lower in subjects who had previously received seasonal influenza vaccination, compared with those who had not. This phenomenon warrants further investigation.