Study Reveals Best Time to Get RSV Vaccine During Pregnancy

New research led by investigators at Mass General Brigham suggests vaccination closer to 32 weeks of pregnancy, rather than the current guidelines of between 32 and 36 weeks, will better protect pregnant women’s newborns against respiratory syncitial virus (RSV). 

In the study that assessed whether maternal vaccine timing is an important consideration for RSV vaccination, the researchers measured RSV antibodies in the umbilical cord at the time of delivery among 124 women who received the RSV vaccine during weeks 32 through 36 of pregnancy and in the blood of 29 2-month-old infants of these mothers. All study participants were receiving care at Massachusetts General Hospital or Mount Sinai Health System in New York City. Levels of RSV antibodies can predict protection against RSV infection in infants too young to yet receive their own vaccines. The researchers found that maternal RSV vaccination at least five weeks prior to delivery led to the most efficient transfer of maternal antibodies across the placenta to the newborn, compared with maternal vaccination at two to three or three to four weeks prior to delivery. 

In an additional analysis, RSV antibody levels in maternal and cord blood after RSV vaccination were compared with RSV antibody levels in 20 unvaccinated mothers. Maternal RSV vaccination resulted in significantly higher and longer-lasting maternal and cord RSV antibody levels.

“This work provides much-needed data to guide physicians in counseling patients about RSV vaccine timing during pregnancy,” said Andrea Edlow, MD, maternal-fetal medicine specialist associate in obstetrics and gynecology in the division of maternal-fetal medicine at Massachusetts General Hospital and associate professor of obstetrics, gynecology and reproductive biology at Harvard Medical School. “Our findings suggest that being vaccinated earlier within the approved timeframe allows for the most efficient placental transfer of antibody to the newborn. They also may have implications for when the RSV monoclonal antibody, Nirsevimab, should be administered to newborns. Similar research should be conducted for other vaccines administered during pregnancy.”

The researchers noted that additional studies are needed to determine the minimum amount of antibody transfer and/or infant blood antibody levels needed to adequately protect infants against RSV. It will also be important to understand the potential additive protection for infants provided by breastmilk from RSV-vaccinated mothers. This study was designed to measure antibody transfer, but larger studies of infants 2 to 6 months of age will be needed to determine the extent to which this leads to enhanced protection.