This study will enroll healthy adults. Participants will receive two doses of the vaccine, 4 weeks apart, and will stay in an isolation facility during both 12-day vaccination periods. Participants will be admitted to the isolation facility 2 days before they receive their first vaccination. They will receive one nasal spray administration of the vaccine and will be monitored for side effects. On various days while in the isolation facility, they will undergo a physical exam, blood and urine collection, and a nasal wash procedure. Participants will remain in the isolation facility for at least 9 days after receiving the vaccine and possibly longer, until the nasal wash tests negative for the vaccine virus for 2 consecutive days. Participants will be readmitted to the isolation facility 26 days after the first vaccination. They will receive a second vaccination and all study procedures will be repeated. Participants will attend study visits 56, 82, and 208 days after the first vaccination for follow-up health and safety monitoring, blood collection, and a nasal wash procedure.