Single doses of heterologous booster vaccine were administered to subjects in the AS03A-adjuvanted H5N1 primary vaccine group at either Month 6, 12, or 36 post-priming; immunogenicity against the primary vaccine strain (A/Vietnam) and the booster strain (A/Indonesia) were assessed after each booster vaccination, and the persistence of immune responses were assessed in all subjects up to Month 48 post-priming.