New study flags potential safety signals in Pfizer’s maternal RSV vaccine

A new study published in JAMA Network Open has identified potential safety signals associated with Pfizer’s maternal respiratory syncytial virus (RSV) vaccine, raising fresh questions about pregnancy-related risks even as researchers stress that their findings are preliminary and do not establish causation.

The study examined Pfizer’s Abrysvo, currently the only RSV vaccine approved in the United States for use during pregnancy. The vaccine is designed to protect infants from birth through six months of age against RSV, a respiratory virus that can cause serious illness in young children.

Elevated Risk Signals Identified

Researchers tracked 13,619 pregnant women who received the RSV vaccine and compared their health outcomes with pregnant women who received influenza, COVID-19, or Tdap vaccines. The analysis drew on post-marketing insurance and health system data from five sources, alongside historical data from vaccinated pregnant women between 2018 and 2023, before RSV vaccines were introduced.

Among the key findings: Vaccinated women were 29% more likely to experience hypertensive disorders of pregnancy, including preeclampsia, eclampsia and HELLP syndrome (Hemolysis, Elevated Liver enzymes and Low Platelets).

Increased hospitalizations for hypertension were observed among vaccine recipients.

The researchers emphasized that the study was designed as an early “sequential surveillance” analysis — a rapid monitoring approach intended to detect potential safety concerns shortly after a vaccine’s introduction. As such, the authors cautioned that their ability to fully account for confounding factors was limited and that the results should be interpreted as preliminary.

No Increase in Preterm Birth Found in This Analysis

Despite concerns raised in earlier research, the JAMA study found no statistically significant increase in preterm births among women who received the RSV shot. This finding was highlighted by both the study authors and media coverage as reassurance amid ongoing debate.

However, previous clinical trials conducted by Pfizer reported higher rates of preterm birth and hypertensive disorders among vaccinated women, though the differences were not statistically significant.

In 2022, GSK halted development of its maternal RSV vaccine candidate after detecting a similar increase in preterm births during trials.

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Additionally, prescribing information for Abrysvo includes warnings about a potential risk of preterm birth and Guillain-Barré syndrome, a rare neurological disorder that was flagged in separate post-marketing research. A study published last year in BMJ Open also reported a higher likelihood of preterm birth among vaccinated pregnant women.

The U.S. Food and Drug Administration approved Abrysvo in August 2023, despite concerns raised by members of its Vaccines and Related Biological Products Advisory Committee (VRBPAC) about preterm birth signals observed during clinical trials.

In September 2023, the Centers for Disease Control and Prevention recommended the vaccine for pregnant women between 32 and 36 weeks of gestation.

During the 2024–2025 RSV season, 31 children and adolescents died from RSV-related illness in the United States. The vaccination status of their mothers was not reported.

The authors of the JAMA study underscored that their findings represent only one phase of an ongoing safety monitoring process. Study co-author Ashley Michnick described the rapid-cycle analysis as an early screening tool meant to guide more comprehensive research rather than deliver definitive risk conclusions.

While the study confidently reported no increased risk of preterm birth in this dataset, it simultaneously identified elevated signals for hypertensive disorders and PROM, prompting calls for deeper investigation.

The study was sponsored by Pfizer, and the authors disclosed receiving research grants from and holding stock options in the company.

As additional studies are planned, health authorities and researchers say ongoing surveillance will be critical in determining whether the identified safety signals reflect true clinical risks or statistical anomalies in early monitoring data.