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When Canada chose an adjuvanted influenza A H1N12009 vaccine for pandemic control, dosing requirements and safety in children were unknown. This study compared responses after 1 and 2 doses in young children.Five Canadian centers evaluated ASO3-adjuvanted H1N12009 vaccine (Arepanrix, GSK Laval, Quebec) in children aged 6 to 35 months, given 2 doses (0.25 mL, 1.9 μg hemagglutinin) 21 days apart. Blood was collected at baseline and 21 to 28 days after each dose to measure hemagglutination inhibition antibody titers. Adverse events were documented daily for 7 days after each dose.Centers enrolled 167 children (mean age, 19 months). At baseline, 29 children (17.4%) had hemagglutination inhibition titers ≥10. After dose 1, titers ≥40 were present in 79.7% of initially naive subjects and 100% of initially primed subjects, whose respective geometric mean titers were 68 and 1020 (P < 0.001). After dose 2, 100% of subjects had titers ≥40, with geometric mean titers increasing to 723 in naive subjects and to 1854 in primed subjects (P < 0.001). Injection site reactions occurred in 56% of subjects after dose 1 and 43% after dose 2 (P < 0.01), with severe reactions noted in ≤3%. Fever >39.0°C was infrequent after either dose (5.4%, 6.6%, P = 0.20). Mild or moderate general symptoms were common but short-lived and less frequent after dose 2.One dose of adjuvanted vaccine met criteria for licensing seasonal influenza vaccines. A second dose increased titers substantially, potentially affording greater or more durable protection. Adverse effects were common but tolerable. The use of adjuvanted seasonal influenza vaccines in young children merits consideration.