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Although children less than 6 months of age have the highest risk for hospitalization related to influenza infection, influenza vaccine is not approved for these children.In an open-label, off-season study, healthy 6 to 12 week and 6-month-old children received 2 doses of the 2004 to 2005 trivalent inactivated influenza vaccine (TIV) administered 1 month apart along with other routine pediatric vaccines. Safety was assessed by parental diaries (n = 393). Immunogenicity analyses (n = 293) were performed on sera obtained before vaccination and 1 month after the second dose of TIV. Outcomes included the frequencies of subjects with injection site and systemic reactions and seroprotection rates to TIV antigens.Injection site reactions and fevers were generally mild and resolved within 3 days. Postvaccination seroprotection rates (titer ≥1:40) in the 6- to 12-week-old and 6-month-old groups were 46% and 69% to A/New Caledonia (H1N1), 59% and 79% to A/Wyoming (H3N2), and 5% and 22% to B/Jiangsu (P < 0.001, all comparisons). For seronegative 6- to 12-week-olds whose mothers had not received TIV during pregnancy, postvaccination seroprotective titers to A/New Caledonia (H1N1) were achieved in 70% (38/54) and to A/Wyoming (H3N2) in 68% (23/34) of infants.TIV was well tolerated and safe when administered to children at both 6 to 12 weeks and 6 months of age. The antibody response was lower in the younger children, probably related to antibody suppression from passively acquired antibodies from mothers. In 6- to 12-week-olds without preexisting antibody, seroresponses to influenza A antigens approached those of 6-month-old children.