Volunteers receive one of two schedules of vaccines or placebo administered at days 0, 28, and 168. Specifically, group 1 receives oral microparticulate monovalent vaccine over 3 consecutive days on days 0, 1, and 2, and 28, 29, and 30, with the intramuscular monovalent vaccine given on day 168. Group 2 receives the intramuscular vaccine first, on day 0, followed by the oral form given on days 28, 29, and 30 and 168, 169, and 170. Volunteers are followed for 1 year.