RSV Vaccine in Pregnancy Not Associated With Increased Preterm Birth Risk, Study Finds

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A new study has found that a vaccine against respiratory syncytial virus (RSV) administered during pregnancy was not associated with increased risks of preterm birth or other poor outcomes. This finding supports the safety of vaccination against RSV, which can cause serious respiratory illness in newborns and infants.

The study was published in JAMA Network Open.

Investigating the vaccine’s safety

RSV can cause severe illness in newborns and young infants. It is one of the leading causes of hospitalizations for infants and young children, causing an estimated 3.2 million hospital admissions among children under 5 year of age worldwide.

In 2023, Pfizer’s RSV vaccine, Abrysvo®, was approved for use in weeks 32–36 of pregnancy to protect infants for 6 months after birth. Although clinical trial data prior to its approval supported the vaccine’s safety, a Phase 3 trial found a 1% higher rate of preterm birth in the vaccination group compared to the control group, limiting the vaccine’s use to after 32 weeks.

Dr. Moeun Son, an associate professor of obstetrics and gynecology at Weill Cornell Medical College, led a new study to investigate these concerns surrounding the vaccine’s safety.

“Using real-world clinical data, we wanted to see if these concerns persisted in a general obstetric population in New York City,” Son explained, speaking to Technology Networks. “We hypothesized that there would be no significant difference in preterm birth in patients who had documented evidence of RSV vaccination during pregnancy and those who did not.”

No association with increased preterm birth risk

The analyzed data from 1,026 vaccinated and 1,947 unvaccinated pregnant patients who gave birth between September 2023 and January 2024.

Preterm birth occurred in 5.9% of vaccinated pregnancies and 6.7% of unvaccinated pregnancies. “Our study showed no increase in the rate of preterm birth among vaccinated pregnant individuals,” said Son.

“We also found that other birth risks were similar between patients who had documented evidence of vaccination and those who did not,” Son explained. These other risks included stillbirths, small-for-gestational-age birthweight, neonatal intensive care unit (NICU) admissions, jaundice, etc.

Additional statistical analysis found a slightly increased risk of high blood pressure during pregnancy in the vaccinated group. However, further investigation is needed to determine whether this is the result of chance or real differences among the two groups. For example, the study also found that those in the vaccinated group were more likely to have health insurance or to have undergone in vitro fertilization.

Overall, the findings are encouraging, as the new study’s use of real-world data – i.e., not data from clinical trial participants, who usually fit strict inclusion criteria – suggests that the vaccine remains safe in a broader range of patients.

“The data are particularly reassuring because we include a broader group of women, including some with health conditions that could increase the risks of poor pregnancy outcomes who were excluded from the clinical trial,” said Son.

Further analysis needed

Nonetheless, Son also highlights that there are important limitations to consider, such as the location of the study population in New York City, which “may not be generalizable to other settings.”

Now, Son and colleagues plan to explore this research area further: “Although the frequency of RSV vaccination was higher at our hospitals than what was reported nationally, prenatal RSV vaccination remains underutilized, and we plan to explore the factors and disparities associated with prenatal RSV vaccination. In addition, potential associations between vaccination and hypertensive disorders of pregnancy need to be further explored in other studies.”

“We hope our findings will reassure pregnant individuals that the RSVpreF (Abrysvo) vaccine is safe and help guide clinicians who are or will provide care to pregnant patients who are set to deliver during the upcoming RSV season,” summarized Son.

Reference: Son M, Riley LE, Staniczenko AP, et al. Nonadjuvanted bivalent respiratory syncytial virus vaccination and perinatal outcomes. JAMA Network Open. 2024;7(7):e2419268. doi: 10.1001/jamanetworkopen.2024.19268

Dr. Moeun Son was speaking to Dr. Sarah Whelan, Science Writer for Technology Networks.

About the interviewee:

Dr. Moeun Son is an associate professor of obstetrics and gynecology at Weill Cornell Medical College. She earned her medical degree from Columbia University College of Physicians and Surgeons and a master’s degree in scientific clinical investigation from Northwestern University Feinberg School of Medicine.