A Randomized, Double-Blind, Placebo-Controlled Study to Evaluate Safety of MEDI-559 in Healthy 1 to <24 Month-Old Children

This is a randomized, double-blind, placebo-controlled, multi-dose, Phase 1/2a multi-center trial to evaluate the safety, tolerability, viral shedding, immunogenicity, and genotypic and phenotypic stability of MEDI-559 in RSV seronegative infants 5 to \<24 months of age and in infants 1 to \<3 months of age regardless of baseline serostatus. The primary objective of this study is to describe the 28-day post-final dose safety and tolerability of three doses of MEDI-559 at 10\^5 FFU when administered to healthy RSV seronegative children 5 to \<24 months of age and to healthy infants 1 to \<3 months of age regardless of baseline serostatus. MEDI-559 will be administered at a dose of 10\^5 fluorescent focus units (FFU) on a 0, 2, and 4 month schedule to two cohorts of subjects in a step-wise fashion. The target sample size for this study is 320 subjects, with 160 subjects 5 to \<24 months of age enrolled into Cohort 1 and 160 subjects 1 to \<3 months of age enrolled into Cohort 2. Each cohort will be randomized 1:1 (MEDI-559 to placebo) and stratified by site. Cohort 1 will initiate dosing at 10\^5 FFU MEDI-559. Blinded safety data from the first 40 subjects enrolled in Cohort 1 for the 28 days following administration of the first dose of vaccine will be reviewed. If no safety concerns are noted, Cohort 2 will initiate dosing at 10\^5 FFU. Enrollment into Cohort 2 will be halted after approximately 40 subjects have been randomized, and blinded safety data will be reviewed through 28 days post Dose 1. If no safety concerns are noted, the remainder of Cohort 2 will be enrolled. All subjects will be followed through 365 days after randomization to ensure that each subject has been followed through an RSV season.