Madhi SA, Polack FP, Piedra PA, et al. - In infants, the dominant cause of severe lower respiratory tract infection is respiratory syncytial virus (RSV), with the most severe cases concentrated among younger infants. In this work, researchers sought to report on the results of a phase 3 trial examining the safety and immunogenicity of RSV F vaccine in pregnant women and vaccine efficacy against RSV-linked lower respiratory tract infection in their infants through the first 90 and 180 days of life. Randomization was performed of a total of 4,636 healthy pregnant women, at 28 weeks 0 days through 36 weeks 0 days of gestation, with an expected delivery date near the start of the RSV season, to receive a single intramuscular dose of RSV fusion (F) protein nanoparticle vaccine or placebo. In the per-protocol analysis, they noted vaccine efficacy of 39% against RSV-associated, medically significant lower respiratory tract infection in the first 90 days of life, during which about three quarters of cases occurred; these outcomes did not satisfy the prespecified criterion for success. In secondary analyses, they noted vaccine efficacy of 48.3% against RSV-associated lower respiratory tract infection with severe hypoxemia and 44.4% against hospitalization for RSV-associated lower respiratory tract infection; these outcomes suggest the potential benefits of maternal RSV vaccination that warrant further study of this strategy.