Among 20 children aged 6-35 months, no evidence was found of antibodies to A (H3N2)v either before or after vaccination with the 2010-11 TIV, whereas 40% or 45% of children demonstrated seroconversion (i.e., a fourfold or greater increase in antibody titer) to the seasonal A (H3N2) virus contained in the vaccine by HI or MN assays, respectively, and 40% of children achieved HI titers of ≥40 and MN titers of ≥80. In contrast, among 30 adults aged 18-49 years, somewhat higher levels of prevaccination antibody to A (H3N2)v were detected, with 33% of this age group achieving HI titers of ≥40 and 43% achieving MN titers of ≥80. The proportion of adults aged 18-49 years with cross-reactive HI and MN antibody to A (H3N2)v increased to 50% and 63%, respectively, after immunization with TIV. As expected, after vaccination, 80% of these adults achieved HI titers of ≥40, and 70% achieved MN titers of ≥80 to the seasonal A (H3N2) vaccine component. Adults aged ≥65 years also exhibited prevaccination antibody to A (H3N2)v, with 17% of 30 achieving HI titers of ≥40 and 30% achieving MN titers ≥80. This increased to 40% postvaccination by either assay. By comparison, 67% and 90% of adults aged ≥65 years exhibited postvaccination HI titers ≥40 or MN titers ≥80, respectively, to the seasonal A (H3N2) vaccine component. Therefore, in these two adult populations, receipt of TIV boosted the levels of antibodies to A (H3N2)v, but to a lesser extent than the antibody response to the A (H3N2) vaccine component.