Abstract After priming with two intramuscular doses of MF59® -adjuvanted (Sub/MF59) or split influenza vaccines during the 2006/07 season, 89 healthy children received a third booster dose of the respective vaccine (2007/08 Northern Hemisphere formulation) approximately 1 year later, and were followed up for 6 months post-third injection. Immunogenicity was evaluated on 81 of them by a hemagglutination inhibition (HI) assay before and 3 weeks after vaccination. The Sub/MF59 influenza vaccine was safe and well tolerated following the booster vaccination. Pre-booster HI antibody titers were consistently higher in the Sub/MF59 group than in the comparator group, confirming significantly longer persistence of antibodies after priming with Sub/MF59 vaccine. Post-booster immune responses were significantly higher in the Sub/MF59 group compared with the split group, especially vs. the influenza B strain, which is epidemiologically relevant in the pediatric population. Altogether, these data further support the potential use of MF59® -adjuvanted influenza vaccine as a safe and highly immunogenic influenza vaccine for young children.