Participants will be randomly assigned to one of three groups and receive either one of the experimental vaccine regimens or a placebo vaccine regimen. Participants will receive four total injections: on Day 0 and Day 28 (DNA vaccine or placebo) and on Day 84 and Day 168 (NYVAC vaccine or placebo). Group 1 participants will receive DNA Nat-B env and NYVAC Nat-B env vaccines, Group 2 participants will receive DNA CON-S env and NYVAC CON-S env vaccines, and Group 3 participants will receive DNA Mosaic env and NYVAC Mosaic env vaccines.