This study is a multicenter, randomized study that is conducted in two parts and comprised of five groups. In all groups, participants will receive four injections. In Part A, Groups 1 and 2 will be compared. In Group 1, participants will receive two shots of the DNA vaccine and two shots of the rMVA vaccine. In Group 2, participants will receive four placebo injections. In Part B, Groups 3, 4, and 5 will be compared. In Group 3, the combination vaccine strategy will be used again; in Group 4, a single-vaccine strategy of three injections of the rMVA vaccine and one injection of placebo will be given; and in Group 5, participants will again receive four placebo injections. The study will last for a total of 12 months for participants, including enrollment and follow-up.