Researchers from Mass General Brigham have discovered that administering the RSV vaccine early in the recommended timeframe significantly enhances maternal antibody transfer to the fetus.
Current health guidelines advise pregnant individuals to get vaccinated against respiratory syncytial virus (RSV)—a virus that generally triggers mild, cold-like symptoms in adults but can be extremely dangerous for infants—between weeks 32 and 36 of their pregnancy. However, new research spearheaded by investigators at Mass General Brigham indicates that receiving the vaccination closer to the 32-week mark could provide optimal protection for newborns against RSV. These important findings are detailed in a study published in the American Journal of Obstetrics & Gynecology.
“Receiving the RSV vaccine in pregnancy is an important way mothers can protect their newborns and infants from RSV, the leading cause of hospitalization in U.S. infants,” emphasized senior author Andrea Edlow, MD MSc, a distinguished maternal-fetal medicine expert at Massachusetts General Hospital, part of the Mass General Brigham healthcare system. “However, it wasn’t clear whether it was equivalent to vaccinate at any time within the approved window, or whether specific weeks were most optimal. Given that the RSV vaccine was ultimately approved for administration during a narrower gestational age window than what was originally studied in large clinical trials, further understanding was crucial regarding how maternal antibodies traverse the placenta from week to week within the approved timeframe.”
Edlow and her team’s prior research on the prenatal administration of COVID-19 mRNA vaccines showcased that the timing of maternal vaccination influences both maternal immune responses and the transplacental transfer of antibodies to the fetus. To explore if the timing of maternal vaccination is equally crucial for RSV vaccination, the researchers analyzed RSV antibody levels in umbilical cord blood at delivery from 124 women who were vaccinated with RSV during the 32–36 week gestation period, along with the blood samples of 29 two-month-old infants born to these mothers. All study participants were receiving care at Massachusetts General Hospital or Mount Sinai Health System in New York City. Higher levels of RSV antibodies in infants are predictive of protection against RSV infection, particularly in those too young to receive their own vaccinations.
The investigators discovered that maternal RSV vaccination occurring at least five weeks before delivery resulted in the most effective transfer of maternal antibodies across the placenta when compared to vaccinations conducted at 2-3 or 3-4 weeks before delivery.
In further analysis, they compared RSV antibody levels in both maternal and cord blood post-vaccination with those in 20 unvaccinated mothers. Results confirmed that maternal RSV vaccination led to noticeably higher and more sustained RSV antibody levels in both mothers and their newborns.
“This work provides much-needed data to guide physicians in counseling patients about RSV vaccine timing during pregnancy,” stated Edlow. “Our findings suggest that vaccination occurring earlier within the approved timeframe allows for the most effective placental transfer of antibodies to the infant. Additionally, this research may influence timing considerations for administering the RSV monoclonal antibody, Nirsevimab, to newborns. Future studies should explore similar questions for other vaccines administered during pregnancy.”
While the investigators acknowledged that further studies are necessary to ascertain the minimum antibody transfer required and the necessary infant blood antibody levels to effectively safeguard infants against RSV, they highlighted the importance of understanding the potential added protection that breastmilk from RSV-vaccinated mothers can offer. This study primarily focused on evaluating antibody transfer, but larger studies involving infants aged 2 to 6 months will be essential to ascertain how this transfer directly equates to improved protection against RSV.
Authorship: In addition to Edlow, Mass General Brigham contributors to the research 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.
Disclosures: Beyond this research, Edlow serves as a consultant for Mirvie, Inc., and has both provided consulting services for and received research funding from Merck Pharmaceuticals. More detailed disclosures can be found within the published paper.
Paper cited: Jasset OJ et al. “Enhanced placental antibody transfer efficiency with longer interval between maternal RSV vaccination and birth” AJOG DOI: 10.1016/j.ajog.2024.10.053
**Interview with Dr. Andrea Edlow, Maternal-Fetal Medicine Expert at Mass General Brigham: Timing Matters for RSV Vaccination during Pregnancy**
**Editor:** Dr. Edlow, thank you for joining us today. Your recent study has shed light on the optimal timing for administering the RSV vaccine during pregnancy. Can you summarize the key findings for our listeners?
**Dr. Edlow:** Thank you for having me. Our research indicates that administering the RSV vaccine closer to the 32-week mark of pregnancy significantly enhances the transfer of maternal antibodies to the fetus. This is crucial because higher levels of these antibodies can help better protect newborns against RSV, which is particularly dangerous for infants and the leading cause of hospitalization in this age group.
**Editor:** That is significant news for expectant mothers. Can you explain why the timing of the vaccine is so important?
**Dr. Edlow:** Absolutely. While current guidelines suggest vaccination between weeks 32 and 36, our findings indicate that getting vaccinated at least five weeks prior to delivery leads to much higher and more effective antibody transfer across the placenta. This means that the earlier mothers receive the vaccine—within that approved window—the better it is for their infants’ protection against RSV.
**Editor:** In your study, you compared women who received the vaccine at different time intervals. What were some of the key differences you observed between those groups?
**Dr. Edlow:** We analyzed umbilical cord blood samples from mothers vaccinated at various intervals and found that those who were vaccinated at 32 weeks had markedly higher levels of RSV antibodies compared to those vaccinated just two to three weeks before delivery. This indicates that timing truly plays a critical role in determining the amount of antibodies passed to the newborn.
**Editor:** Your research also involved comparisons with unvaccinated mothers. What did those findings reveal?
**Dr. Edlow:** The comparison showed that mothers who received the RSV vaccine had significantly higher and more sustained antibody levels than unvaccinated mothers. This reinforces the importance of vaccination not only for maternal health but also for providing early and stronger immunity to newborns against RSV.
**Editor:** With these insights, how should healthcare providers adjust their recommendations regarding RSV vaccination for pregnant individuals?
**Dr. Edlow:** This research provides concrete data that can guide physicians in counseling pregnant patients about the timing of the RSV vaccine. It’s essential for healthcare providers to encourage vaccination early—in the 32 to 36-week window—to optimize the protective effects for both mothers and their infants.
**Editor:** Thank you, Dr. Edlow, for sharing these important findings with us today. It’s clear that timely vaccination can make a substantial difference in protecting newborns against RSV.
**Dr. Edlow:** Thank you for having me. It’s vital that we continue to disseminate this information to help improve health outcomes for mothers and their infants.