Study Shows the Best Time to Administer RSV Vaccine During Pregnancy to Protect Newborns

Current guidelines recommend that pregnant women receive a vaccination at weeks 32 through 36 of pregnancy against respiratory syncytial virus (RSV), which usually causes mild, cold-like symptoms in most adults but can be fatal to infants. New research led by Mass General Brigham researchers suggests that vaccination earlier in this period, closer to 32 weeks, may provide the best protection for newborns against RSV. The findings were published in the American Journal of Obstetrics & Gynecology.

RSV Vaccination of the Mother at Least Five Weeks Before Delivery Leads to More Efficient Transfer of Maternal Antibodies to the Baby

“RSV vaccination during pregnancy is an important way for mothers to protect their newborns and infants from RSV, the leading cause of infant hospitalization in the United States,” said lead author Andrea Edlow, MD MSc, a maternal-fetal medicine specialist in the Department of Obstetrics and Gynecology at Massachusetts General Hospital, a founding member of the Mass General Brigham Health System. “However, it was not clear whether it was equivalent to vaccinate at any time within the approved window, or whether specific weeks were most optimal. Because the RSV vaccine was ultimately approved for administration in a narrower gestational window than was originally studied in the large clinical trial, more information was needed about how maternal antibodies spread across the placenta week by week across the approved window.”

Edlow and her team had previously studied prenatal administration of the COVID-19 mRNA vaccines and found that the timing of maternal vaccination was associated with altered maternal responses and transplacental antibody transfer to the fetus. To assess whether the timing of maternal vaccination also plays an important role in RSV vaccination, the researchers measured RSV antibodies in the umbilical cord at the time of delivery in 124 women who had received the RSV vaccine at 32 to 36 weeks’ gestation and in the blood of 29 two-month-old infants born to these mothers. All study participants were cared for at Massachusetts General Hospital or Mount Sinai Health System in New York City. RSV antibody levels can predict protection against RSV infection in infants who are too young to receive their own vaccines. The researchers found that RSV vaccination of the mother at least five weeks before delivery resulted in more efficient transfer of maternal antibodies across the placenta to the newborn compared to vaccination of the mother two to three or three to four weeks before delivery.

Further Research Needed

In an additional analysis, RSV antibody levels in maternal blood and umbilical cord blood after RSV vaccination were compared with RSV antibody levels in 20 unvaccinated mothers. RSV vaccination of the mother led to significantly higher and longer-lasting RSV antibody levels in both mother and child. “This work provides much-needed data that can guide physicians when advising patients on the timing of RSV vaccination during pregnancy,” said Edlow. According to the researchers, the results suggest that earlier vaccination within the approved time frame allows for the most efficient placental antibody transfer to the newborn. They could also have implications for when the RSV monoclonal antibody nirsevimab should be administered to newborns. Similar studies should be conducted for other vaccines administered during pregnancy.

The researchers noted that further studies are needed to determine the minimum amount of antibody transfer and/or antibodies in the infant’s blood required to adequately protect infants against RSV. It will also be important to understand the potential additional protection provided to infants by breast milk from RSV-vaccinated mothers. This study was designed to measure antibody transfer, but larger studies in infants aged 2 to 6 months will be needed to determine the extent to which this leads to improved protection.