• Posted July 9, 2024

Study Shows RSV Vaccine Safe in Late Pregnancy

Expectant mothers who get vaccinated to protect their newborns against respiratory syncytial virus (RSV) are not putting themselves or their babies at risk, new research affirms.

It found that getting the shot during late pregnancy was not associated with increased odds of preterm birth or other outcomes.

The difference in preterm birth rates between vaccinated women (5.9%) and unvaccinated women (6.7%) was not statistically significant, researchers reported July 8 in the journal JAMA Network Open. The finding adds to clinical trial data about the safety of Pfizer's Abrysvo vaccine.

"The real-world evidence provides an additional layer of confidence about the safety of this vaccine during pregnancy," said lead study author Dr. Moeun Son, an associate professor of obstetrics and gynecology at Weill Cornell Medicine in New York City. "We now have data from multiple populations showing no increase in preterm birth risk."

Up to 80,000 children under age 5 wind up in the hospital nationwide each year with RSV infections, and up to 300 die, according to the U.S. Centers for Disease Control and Prevention. 

Based on clinical trial findings, the U.S. Food and Drug Administration approved Abrysvo last August for women between 32 and 36 weeks of pregnancy. 

A CDC panel then recommended the shot for eligible pregnant women during RSV season, which runs from September through January.

The shot, which is given in a single dose, stimulates production of protective antibodies that pass from the mother to the fetus through the placenta. 

For the new study, Son and her team analyzed pregnancy outcomes in 1,026 vaccinated and 1,947 unvaccinated women. They were treated at two affiliated New York City hospitals.

Researchers looked at rates of preterm birth, as well as stillbirths, low birth weight, admission to newborn intensive care units (NICU), respiratory distress in NICU babies, low blood sugar and rates of a serious complication called sepsis. 

"Patients and clinicians can feel confident that vaccination during pregnancy is a safe way to protect infants from harmful RSV infections," said Son, who is a maternal-fetal medicine doctor at New York-Presbyterian/Weill Cornell Medical Center.

To verify the results, her team did three separate analyses using different statistical methods. In one, they did notice a slightly higher risk of high blood pressure during pregnancy in women who were vaccinated. 

More study will be needed to determine if that is a genuine concern or a result of chance or differences between the two groups of women, the researchers said.

"These are things we will continue to explore in future studies," Son said in a Weill Cornell Medicine news release.

The two hospitals in the study vaccinated 35% of pregnant women in their care -- almost double the national average of 18%. Vaccinated women were more likely to be insured or to have undergone in vitro fertilization. Black women, those with public insurance and those who had fewer prenatal visits were less likely to be immunized, the study found.

The next step, Son said, is to speak to different communities to understand why some women are hesitant to get the shot or what obstacles they might face. Researchers hope to do so before RSV season begins in September.

"We want to ensure that all who would benefit will receive the vaccine," she said.

More information

Find out if you need to be vaccinated for respiratory syncytial virus at the U.S. Centers for Disease Control and Prevention.

SOURCE: Weill Cornell Medicine, news release, July 8, 2024