Evaluating the Infectivity, Safety and Immunogenicity of Respiratory Syncytial Virus Vaccines, RSV 6120/∆NS1 and RSV 6120/F1/G2/∆NS1, in RSV-Seropositive Children and RSV-Seronegative Infants and Children

This study will evaluate the infectivity, safety, and immunogenicity of a single dose of recombinant, live-attenuated respiratory syncytial virus (RSV) vaccines (RSV 6120/∆NS1 or RSV 6120/F1/G2/∆NS1) in RSV-seropositive children 12 to 59 months of age and RSV-seronegative infants and children 6 to 24 months of age. The vaccines will be evaluated in a stepwise fashion beginning with RSV-seropositive children (Group 1) and proceeding sequentially in RSV-seronegative infants and children (Group 2). In each group, participants will be randomly assigned to receive a single dose of RSV 6120/∆NS1, RSV 6120/F1/G2/∆NS1, or placebo at Day 0. Participants will be enrolled in the study between April 1 and October 31, outside of the RSV season. Group 1 participants will be followed for 28 days after inoculation, and Group 2 participants will remain on the study until they complete the post-RSV season visit between April 1 and April 30 in the calendar year following enrollment. The expected duration of the initial study is 28 days for Group 1 participants and between 6 and 13 months for Group 2 participants, depending upon time of enrollment. Study visits for all participants may include clinical assessments, blood collection, and nasal washes or nasal swabs. Additionally, participants' parents or guardians will be contacted by study staff at various times during the study to monitor participants' health.