Trivalent and quadrivalent MF59®-adjuvanted influenza vaccine in young children: A dose- and schedule-finding study

► This study confirms that the immunogenicity of nonadjuvanted influenza vaccines is suboptimal in 6-36 month-old children. ► Addition of MF59® adjuvant increased antibody responses to levels associated with protection in adults to an extent not achieved by nonadjuvanted vaccines and with no impact on reactogenicity and safety in these young children. ► A second influenza B strain combined with the traditional TIV vaccine is immunogenic and does not affect immunogenicity of the other three influenza strains. ► The MF59-adjuvanted TIV and QIV vaccines already show a meaningful immune response to influenza A strains after one dose, which may be beneficial in real-life clinical practice where a second dose is often missed, but the two-dose vaccination schedule must continue to be recommended in young children to provide protection against influenza B strains.