This study will evaluate the safety and immunogenicity of a single dose of the recombinant live-attenuated respiratory syncytial virus (RSV) vaccines, RSV ΔNS2/Δ1313/I1314L, RSV 6120/ΔNS2/1030s, and RSV 276, in RSV-seronegative children 6 to 24 months of age.~Participants will be randomly assigned to receive a single dose of RSV ΔNS2/Δ1313/I1314L vaccine, RSV 6120/ΔNS2/1030s vaccine, RSV 276 vaccine, or placebo intranasally at study entry.~Participants will be enrolled in the study outside of RSV season. All participants will remain on study until they complete the post-RSV season visit between April 1 and April 30 in the calendar year following enrollment. Participants' total study duration will be between 6 and 13 months, depending on when they enroll in the study.~Participants will attend several study visits throughout the study, which may include physical examinations, blood collection, and nasal washes. Participants' parents or guardians will be contacted by study staff at various times during the study to monitor participants' health.
The purpose of this study is to evaluate the safety and immunogenicity of a single dose of the recombinant live-attenuated respiratory syncytial virus (RSV) vaccines, RSV ΔNS2/Δ1313/I1314L, RSV 6120/ΔNS2/1030s, and RSV 276, in RSV-seronegative children 6 to 24 months of age.