Evaluating the Infectivity, Safety, and Immunogenicity of Recombinant Live-Attenuated RSV Vaccines RSV ΔNS2/Δ1313/I1314L or RSV 276 in RSV-Seronegative Infants 6 to 24 Months of Age

Human respiratory syncytial virus (RSV) is the most common viral cause of serious acute lower respiratory illness (LRI) in infants and children under 5 years of age worldwide. This study evaluated the infectivity, safety, and immunogenicity of two recombinant live-attenuated RSV vaccines: RSV ΔNS2/Δ1313/I1314L and RSV 276. The vaccines were delivered as nose drops to RSV-seronegative infants 6 to 24 months of age. Participants were randomly assigned to receive a single dose of the RSV ΔNS2/Δ1313/I1314L vaccine, the RSV 276 vaccine, or placebo at study entry (Day 0). Participants could be enrolled in the study outside of RSV season, i.e., between April 1 and October 14 for most sites or-for sites with local RSV seasons that start earlier-as specified on a site-by-site basis in the manual of procedures. Participants remained on study until they completed the post-RSV season visit between April 1 and April 30 in the calendar year following enrollment. Participants' total study duration was between 6 and 13 months, depending on when they enrolled in the study. Participants attended several study visits throughout the study, which included physical examinations, blood collection, nasal washes, and/or nasal adsorption (nasosorption) specimen collection. Participants' parents or guardians were contacted by study staff at various times during the study to monitor participants' health.