Safety of a Respiratory Syncytial Virus (RSV) Vaccine in Pregnant Women and their Infants. Final Report: A Phase 2b, Randomized, Placebo-Controlled, Observer-Blinded Trial to Evaluate the Safety, Tolerability, and Immunogenicity of a Respiratory Syncytial Virus (RSV) Vaccine in Pregnant Women 18 Through 49 Years of Age and Their Infants.