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Respiratory syncytial virus (RSV) infections during the first 6 months of life cause substantial morbidity in developed countries and mortality in low- and middle-income countries (LMIC). To test the efficacy of a prefusion-stabilized F protein–based vaccine in pregnant women for preventing medically attended RSV-associated lower respiratory tract disease in their infants, investigators planned to enroll 10,000 pregnant women in a multinational, industry-funded study. After 5328 participants were enrolled, they were randomized to receive vaccine (3551 women) or placebo (1771 women); however, the study was halted early due to an imbalance of preterm births between groups.
In the vaccine and placebo groups, maternal gestation at vaccination wa…