To determine study eligibility, the screening process will include a blood collection. Screening may begin after the RSV season (i.e., as of April 1) and enrollment must precede the RSV season (no later than October 14). At study entry, eligible participants will be randomly assigned to receive one dose of either the RSV vaccine or placebo, which will be delivered as nose drops. Participants will also undergo a review of medical history, clinical assessment, and a nasal wash. They will then receive their assigned vaccine and will remain under observation for monitoring for 30 minutes after receiving the vaccine. Additional study visits will occur at Days 3, 5, 7, 10, 12, 14, 17, 19, 21, 28, and 56. These visits will include clinical assessments and nasal washes; on Day 56, a blood collection will also occur. On days where no study visit is scheduled (through Day 29), participants' parents or guardians will report participants' temperatures and signs of illness to researchers by e-mail or phone.