An analysis of safety data suggests that Abrysvo, the Pfizer respiratory syncytial virus (RSV) RSVpreF vaccine, does not increase the risk of adverse pregnancy outcomes when administered after 32 weeks’ gestation, according to a study published yesterday in JAMA.

RSV is a leading cause of illness and death in infants across the globe, and maternal vaccination during pregnancy is intended to protect newborns during their first months of life. Previous clinical trials suggested that the vaccine was effective at conferring protection, but small increases in preterm birth and hypertensive disorders during pregnancy prompted calls for continued safety monitoring.

No statistically significant increase in preterm birth

The analysis, led by Harvard Medical School researchers and including authors from Pfizer, draws on data from five US health plans on 54,011 pregnancies during the first full RSV season following Abrysvo’s approval (September 2023 to June 2024). 

Of all participants, 19% received the Abrysvo vaccine from 32 to 36 weeks’ gestation. The team found no statistically significant increase in preterm birth, hypertensive (high blood pressure) disorders, premature rupture of membranes (PROM), or preterm PROM among vaccinated women.

Previous safety analyses found a statistically significant increased risk of hypertensive disorders associated with the vaccine, and one clinical trial for an RSV vaccine from a different manufacturer administered from 24 to 34 weeks gestation was stopped early because of an increased risk of preterm birth. 

But, note the researchers, the current study better controlled for confounding factors and corroborated previous findings that suggested no increased risk of preterm birth associated with Abrysvo. 

Safety monitoring is ongoing. “Continual safety monitoring is crucial,” write the authors, who will issue a final safety report in 2029 that will assess more outcomes, include underrepresented populations, and analyze different subgroups.