Recent studies have found that children <5 years of age are less well protected than older individuals even though their serum antibody response rates (take rates) were similar to the older subjects when the serum was collected about two weeks after the vaccine doses were administered. These immunogenicity studies have not, however, examined antibody titers when blood samples were obtained after a longer time interval. Therefore, it is not known if the antibody titers decline at the same rate as older subjects. It could be that GMT titers are lower or that titers fall more rapidly in the young children, and a different dosing interval could maintain higher titres. To compare antibody responses in different age groups, we plan to stratify groups into age 1-4, 5-14, and 15 years and older.