Immunogenicity of heterologous H5N1 influenza booster vaccination 6 or 18 months after primary vaccination in adults: a randomized controlled clinical trial.

•We compared an AS03-adjuvanted H5N1 vaccine at different dose levels to placebo.•We compared the effect of a vaccine boost 6 or 18 months later with a different strain than the priming strain.•Boosting 6 or 18 months later results in rapid and durable immune responses.