Safety and Immunogenicity of an Investigational Respiratory Syncytial Virus Vaccine (RSVPreF3) in Mothers and Their Infants: A Phase 2 Randomized Trial

Lay Summary What Is the Context? Infants, especially those less than 6 months of age, are at increased risk of lung infection caused by respiratory syncytial virus (RSV). However, this risk could be reduced with maternal vaccination against RSV during pregnancy. A previous clinical trial found that a vaccine candidate (named RSVPreF3) was well tolerated when given to non-pregnant women. What is New? In pregnant women, RSVPreF3 was also well tolerated. Occurrence of unsolicited adverse events was similar between vaccine and placebo recipients. None of the serious adverse events or events of interest for pregnant women or newborns were considered related to the study intervention. One month after vaccination, mothers who received RSVPreF3 had 11-15 times higher levels of antibodies against RSV than before vaccination. These antibody levels remained similar until 43 days after delivery. In the infants born to mothers vaccinated during pregnancy with RSVPreF3, antibody levels were highest at birth, when levels were higher than in their mothers, and declined through day 181 postbirth. What Is the Impact? RSVPreF3 had an acceptable safety risk profile in pregnant women and their babies. This vaccine induced potent immune responses against RSV, with maternal antibodies transferred to infants of the vaccinated mothers. Background In a phase 1/2 study, a maternal respiratory syncytial virus vaccine candidate (RSVPreF3) demonstrated an acceptable safety profile and efficiently increased RSV-specific humoral immune responses in non-pregnant women. Methods In this phase 2 observer-blind, placebo-controlled, randomized clinical trial (NCT04126213), the safety of RSVPreF3 (60 or 120 mu g), administered during late second or third trimester, was evaluated in 213 18- to 40-year-old healthy pregnant women through 6 months postdelivery and their offspring through infancy; immunogenicity was evaluated through day 43 postdelivery and day 181 postbirth, respectively. Results RSVPreF3 was well tolerated. No pregnancy-related or neonatal adverse events of special interest were considered vaccine/placebo related. In the 60 and 120 mu g RSVPreF3 groups: (1) neutralizing antibody (nAb) titers in mothers increased 12.7- and 14.9-fold against RSV-A and 10.6- and 13.2-fold against RSV-B, respectively, 1 month postvaccination and remained 8.9-10.0-fold over prevaccination at day 43 postdelivery; (2) nAb titers were consistently higher compared to placebo recipients; (3) placental transfer ratios for anti-RSVPreF3 antibodies at birth were 1.62 and 1.90, respectively, and (4) nAb levels in infants were highest at birth and declined through day 181 postbirth. Conclusions RSVPreF3 maternal vaccination had an acceptable safety risk profile and induced robust RSV-specific immune responses with successful antibody transfer to their newborns. In this phase 2 observer-blind, placebo-controlled, randomized clinical trial, RSVPreF3 maternal vaccination during late second or third trimester had an acceptable safety risk profile and induced robust RSV-specific immune responses with successful antibody transfer to their newborns.