To better protect newborns from respiratory syncytial virus (RSV), the leading cause of hospitalization in U.S. infants, pregnant women should receive a vaccine five weeks before delivery, according to new research led by investigators at Mass General Brigham.

RSV typically causes mild, cold-like symptoms in most adults but can be deadly for infants. While current guidelines recommend a vaccine during weeks 32–36 of pregnancy, new findings suggest that vaccination closer to 32 weeks could provide the best protection. Results of the study are published in the American Journal of Obstetrics & Gynecology.

To assess whether maternal vaccine timing is an important consideration for RSV vaccination, the investigators measured RSV antibodies in the umbilical cord at the time of delivery among 124 women who received the RSV vaccine during weeks 32–36 of pregnancy and in the blood of 29 2-month-old infants of these mothers.

All study participants were receiving care at MGH 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 investigators found that maternal RSV vaccination at least five weeks before 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.

“Our findings suggest that being vaccinated earlier within the approved timeframe allows for the most efficient placental transfer of antibody to the newborn,” said senior author Andrea Edlow, a maternal-fetal medicine specialist in the Department of Obstetrics and Gynecology at Massachusetts General Hospital. “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.

“This work provides much-needed data to guide physicians in counseling patients about RSV vaccine timing during pregnancy,” Edlow added.

The investigators noted that additional studies are needed to determine the minimum amount of antibody transfer and/or infant blood antibody levels 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.

Disclosures: Outside of this work, Edlow serves as a consultant for Mirvie, Inc. and is a consultant for and has received research funding from Merck Pharmaceuticals. Additional disclosures can be found in the paper.

Authorship: In addition to Edlow, Mass General Brigham authors include Olyvia J. Jasset, Paola Andrea Lopez Zapana, Lydia Shook, Emily Gilbert, Zhaojing Ariel Liu, Rachel V. Yinger, Caroline Bald, Caroline G. Bradford, Alexa H. Silfen, and Lael M. Yonker.

This work was funded by National Institute of Allergy and Infectious Disease (1U19AI167899, R01AI171980), the National Institute of Child Health and Human Development (5K12HD103096 to L.L.S; NIH/NHLBI: R01HL173059 to L.Y.; MGH ECOR: MGH Research Scholar Award to A.G.E., Claflin Award to L.L.S.; Binational Science Foundation Award number 2019075 to L.K.) None of the funders had any role in the design of the study; in the collection, analysis, and interpretation of data; in the writing of the report; or in the decision to submit the article for publication.