Upon returning to clinic on day 28 after dose 2 (day 56 after dose 1), the child will receive an unblinded, "insurance", full-volume, 0.25 mL dose (#3) of influenza vaccine by NS IM, unless he or she is in the full-dose IM control group IM-NS-0.25, in which case a mock injection will be administered instead of a 3rd full dose beyond the usual 2-dose series. All participants will return 6 months after this third injection for a fourth "bonus" dose of influenza vaccine to ensure protection for the following season.