Late-pregnancy RSV vaccination not associated with poor birth outcomes


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Babies exposed to maternal RSV (respiratory syncytial virus) vaccination in the third trimester are not at an increased risk of preterm birth or other poor birth outcomes, according to a study today in JAMA Network Open. 

The US Food and Drug Administration approved Pfizer’s Abrysvo vaccine for use in women 32 to 36 weeks pregnant last August based on clinical trial results. Maternal vaccination in late pregnancy is meant to pass protective antibodies on to the newborn, but clinical trial results showing an increased risk in preterm birth shortened the recommended vaccination window from 24 to 36 weeks to 32 to 36 weeks. 

Each year, RSV causes 80,000 hospitalizations and up to 300 deaths in children under age 5 in the United States. The Centers for Disease Control and Prevention recommends that pregnant women receive one RSV vaccine dose from September to January to protect infants against the virus. 

No significant differences in preterm birth rates

In today's study, the vaccine was found to be safe and protective in a real-world setting, which included 1,026 vaccinated and 1,947 unvaccinated patients who received care at two New York City hospitals from September 22, 2023, to January 31, 2024. Preterm birth was defined as birth before 37 weeks' gestation. 

Sixty patients who had evidence of prenatal vaccination (5.9%) experienced preterm birth, compared with 6.7% of those who did not.

"The real-world evidence provides an additional layer of confidence about the safety of this vaccine during pregnancy," said lead author Moeun Son, MD, associate professor of obstetrics and gynecology at Weill Cornell Medicine in a press release from Weill Cornell. "Randomized clinical trials don’t always emulate the populations we see in the clinical setting, but now we have data from multiple populations showing no increase in preterm birth risk."

The real-world evidence provides an additional layer of confidence about the safety of this vaccine during pregnancy.

There were no significant differences in rates of small-for-gestational-age (SGA) births or stillbirth, but there was an increased risk of overall hypertension disorders in pregnancy with RSV vaccination in a time-dependent model (hazard ratio, 1.43; 95% confidence interval, 1.16 to 1.77).

Second vaccination season looming

In an editorial on the study, Annette Regan, PhD, MPH, of the University of San Francisco, writes that the findings should reassure clinicians and women that the vaccine is safe, especially after phase 3 clinical data raised concerns regarding a 1% higher rate of preterm birth observed in the intervention arm compared with the control arm. 

"Despite the need for further research and surveillance, results from this study should offer initial reassurance to regulators, policymakers, health care professionals, and pregnant patients—as well as additional countries considering the introduction of RSVpreF vaccine programs for pregnant people," Regan writes.

Son said the study should help guide clinicians who are currently providing care for women set to deliver during RSV season.

"As we prepare for the next RSV vaccination season in September, we are going to speak with different communities to help understand what might cause vaccine hesitancy among women or create barriers to access," Son said. "We want to ensure that all who would benefit will receive the vaccine." 


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