This study will evaluate the infectivity, safety, and immunogenicity of a single dose of a recombinant, live-attenuated RSV vaccine (RSV 6120/∆NS2/1030s) in RSV-seropositive children 12 to 59 months of age and RSV-seronegative infants and children 6 to 24 months of age.~The vaccine will be evaluated in a stepwise fashion beginning in RSV-seropositive children (Group 1), and then in RSV-seronegative infants and children (Group 2). In each group, participants will be randomly assigned to receive a single dose of RSV 6120/∆NS2/1030s vaccine or placebo at study entry (Day 0).~Participants will be enrolled in the study between April 1 and October 31, outside of the RSV season. Group 1 (RSV-seropositive children) will be followed for 28 days. Group 2 (RSV-seronegative infants and children) will remain on study until they complete the post-RSV season visit.~Group 2 participants may participate in an optional second season of RSV surveillance during November to March of the second year following enrollment.~Study visits for all participants may include clinical assessments, blood collection, and nasal washes. Additionally, 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 infectivity, safety, and immunogenicity of a single dose of a recombinant, live-attenuated respiratory syncytial virus (RSV) vaccine (RSV 6120/∆NS2/1030s) in RSV-seropositive children 12 to 59 months of age and RSV-seronegative infants and children 6 to 24 months of age.