This is a randomized, double-blind, placebo-controlled, dose-escalation, Phase 1, multi-center study to evaluate the safety, tolerability, immunogenicity, and viral shedding of MEDI-534 in healthy children 1-9 years of age who are seropositive for respiratory syncytial virus (RSV) and parainfluenza virus type 3 (PIV3) at screening.

This is a randomized, double-blind, placebo-controlled, dose-escalation Phase 1 multi-center study to evaluate the safety, tolerability, immunogenicity, and viral shedding of MEDI-534 in healthy children 1-9 years of age who are seropositive for RSV and PIV3 at screening. MEDI-534 will be administered at dosage levels of 10:4 TCID50, 10:5 TCID50, or 10:6 TCID50 to three cohorts of subjects in a staggered, step-wise fashion. A single dose of study vaccine (MEDI-534 or matched volume of vehicle placebo) will be administered on Study Day 0 by nasal spray, one-half dose into each nostril. The target sample size is 120 subjects randomized 1:1 (MEDI-534 to placebo), with 40 subjects in each of three cohorts. Randomization will be stratified by site. This study will enroll during the RSV off season at multiple sites in the United States and Chile.