Depending on their order of entry into the study, participants are assigned to receive one of three vaccine doses or placebo. The first group receives the lowest dose (2 milligram) of vaccine or placebo. If this dose is safe, then the second group receives 4 mg, and if this dose is safe, the third group receives 8 mg. Injections are given in a muscle in the upper arm. Participants receive three injections, each 4 weeks apart (on study days 0, 28, and 56).