Abstract
BACKGROUND: There is a need for better vaccines and vaccine strategies to reduce the burden of influenza in very young children. METHODS: This phase 1, open-label study assessed the reactogenicity, safety, and immunogenicity of an inactivated trivalent influenza vaccine (TIV) containing low doses of hemagglutinin antigen (7.5 µg each strain), adjuvanted with a tocopherol-based oil-in-water emulsion Adjuvant System (AS03). Influenza vaccine-naïve children aged 6-35 months were sequentially enrolled to receive TIV-AS03D (1.48 mg tocopherol) or TIV-AS03C (2.97 mg tocopherol), then a 6-month booster of conventional TIV. The primary endpoint was the incidence of fever (axillary temperature >38 °C) for 7 days post-vaccination. Immune responses were assessed by hemagglutination-inhibition (HI) assay. RESULTS: Forty children were sequentially enrolled into the TIV-AS03D or the TIV-AS03C group. Fever >38.0 °C was reported in 5/20 (25.0%) and 7/20 (35.0%) children after the first and second doses of TIV-AS03D, respectively, and in 7/20 (35.0%) children after 1 dose of TIV-AS03C; the latter fulfilled the holding rule for safety, and the second dose of TIV-AS03C was cancelled. HI immune responses exceeded adult European licensure criteria for the immunogenicity, and all children had HI antibody titers ≥ 1:40 after 1 dose of TIV booster against booster strains. CONCLUSIONS: One dose of primary vaccine containing a low dose of antigen and AS03 may be a possible influenza vaccination strategy for young children. The relatively high frequency of fever warrants further investigation, although the generalizability of the findings are uncertain given that many of the children had antibody evidence suggesting recent infection with A(H1N1)pdm09.