Each winter, viruses belonging to two kinds of influenza A ("A/H1N1" & "A/H3N2") and two kinds of influenza B ("B/Yamagata" & "B/Victoria") can cause illness. The yearly influenza vaccine is designed to protect against both kinds of influenza A but only one or the other kind of influenza B. The vaccine is changed from year to year, meaning it may include one kind of B virus one year and the other kind another year. But because influenza is so hard to predict, sometimes the kind of B virus chosen for the vaccine may not match the kind that is causing illness. The National Advisory Committee on Immunization recommends that all infants and toddlers receive influenza vaccine to protect against their high rates of hospitalization. Infants/toddlers receiving influenza vaccine for the first time must get two doses (prime plus boost) to have a good antibody response. If they have ever before received a single dose of influenza vaccine, then they are recommended to receive only one dose each year afterwards. But we don't know how well previous doses of one kind of influenza B set the stage for good antibody response to a single dose of the other kind of influenza B. This study will try to answer that question in a group of infants/toddlers who last year received two doses of one kind of B virus ("Yamagata"), as part of another study. This year, we will give them a single dose of influenza vaccine that now contains the other kind of B virus ("Victoria") and see how much antibody they make to both kinds. About half these children received a higher amount of influenza vaccine in the previous year's study, so we will also compare their antibody levels on that basis. Since influenza B is an illness especially of children, understanding how to best protect infants/toddlers against both kinds of influenza B is important. This study will help us know if we need to design a new vaccine that not only includes both kinds of influenza A, but also both kinds of influenza B.