At the screening visit, participants will give a medical history and undergo a complete physical exam, electrocardiogram (ECG), urine collection, blood collection, interview, HIV test, and pregnancy test (for participants who were born female). Participants will receive an intramuscular (IM) vaccination (study vaccine or placebo) into the deltoid on the schedule assigned to their group. The vaccination schedule is as follows: Group 1 participants will receive an injection on Days 0, 56, 112, 168, and 224; Group 2 participants will receive an injection on Days 0, 56, 112, 168, and 303; Group 3 participants will receive an injection on Days 0, 56, 112, and 224. On vaccination visits, participants will also undergo an abbreviated physical exam, a pregnancy test (for participants who were born female), risk-reduction counseling, and blood collection. Immediately following vaccination, participants will remain in the clinic for observation for 30 minutes; participants will be given a post-vaccination symptom log and instructed on how to complete it. Follow-up visits will consist of a brief physical exam, blood collection, and interview; some follow-up visits may also consist of a urine collection, HIV test, or ECG.