Interim Analysis Shows No Safety Concerns for RSV Vaccine During Pregnancy
Key Insights
An interim analysis of 54,011 pregnancies found no statistically significant increases in key pregnancy complications among individuals who received RSVpreF (Abrysvo) during pregnancy.
The study matched 6,857 vaccinated pregnancies with 6,857 unvaccinated pregnancies and found no increased risks for preterm birth (search), hypertensive disorders, or membrane rupture complications.
These real-world safety findings provide reassurance for clinicians and patients, though final study results are planned for 2029 as part of ongoing safety surveillance.
An interim analysis published in JAMA on January 8, 2026, found no statistically significant increases in key pregnancy complications among individuals who received the RSVpreF (Abrysvo; Pfizer) during pregnancy. The study assessed 54,011 pregnancies across 5 US health plans, providing the first comprehensive real-world safety data during the first full RSV (search) season after vaccine approval.
Study Design and Population
The research evaluated pregnancies from September 22, 2023, through June 14, 2024, of which 10,273 (19.0%) received RSVpreF between 32 weeks 0 days and 36 weeks 6 days' gestation. After 1:1 matching, 6,857 vaccinated pregnancies were compared with 6,857 unvaccinated pregnancies matched on gestational age, calendar quarter, baseline vaccine-seeking behavior, and key clinical factors.
"From essentially large health plans of primarily commercially insured pregnant individuals in the United States, we matched those who received the RSVpreF vaccination during pregnancy…to those who did not receive that vaccine," said Ashley Michnick, PharmD, PhD, research associate at Harvard Medical School's Department of Population Medicine and corresponding author of the research letter. "For none of the outcomes that we were looking at did we find any increased risks."
Safety Outcomes
After adjustment using Cox proportional hazards models, researchers found no statistically significant increases in preterm birth (search) (4.2% vs. 5.5%), hypertensive disorders of pregnancy (search) (14.3% vs. 13.1%), premature rupture of membranes (search) (14% vs. 13.5%), or preterm PROM (1.8% vs. 2.3%) among vaccinated versus unvaccinated women. The mean maternal age at delivery was 33 years, and most pregnancies were among individuals of White or unknown race.
Methodological Rigor
The study employed robust matching strategies to reduce confounding, including consideration of prior hypertensive disorders, immunocompromising conditions, and baseline likelihood of vaccination. This approach addressed limitations identified in earlier surveillance analyses.
"When we use real-world data like we did in this study, there can be differences between patients who chose to be vaccinated and those who didn't," Michnick explained. "Sometimes these differences have to do with underlying health factors that could actually influence the safety profile of the vaccine. So we were really careful in this study to do our absolute best to measure those potential confounding factors."
Context with Previous Findings
The findings differ from earlier surveillance signals that reported small but statistically significant increases in hypertensive disorders of pregnancy (search). Prior Vaccine Safety Datalink analyses showed adjusted relative risk of 1.14 and hazard ratio of 1.09 for hypertensive disorders, though those studies had less robust control for confounding compared to the current analysis.
Ongoing Surveillance Framework
Sarah MacDonald, ScD, ScM, epidemiologist and director of safety surveillance research at Pfizer, emphasized the importance of continuous monitoring. "As part of our commitment to patient safety, all Pfizer products are monitored continuously for safety before and after approval," she told Healio. "This paper shares early findings from one of our ongoing studies that continuously monitors the safety of Abrysvo."
Michnick noted that this represents only an early snapshot. "This is an interim report of an ongoing cohort study, and even that ongoing cohort study is just one part of a larger safety surveillance framework for this vaccination during pregnancy," she said. Final results are planned for 2029.
Clinical Implications
The FDA approved Abrysvo in August 2023 based on data showing it prevented RSV (search) in infants during their first 6 months after birth. The vaccine is administered in a single dose between weeks 32 and 36 of pregnancy.
"Practitioners can reassure their patients that we know what people are concerned about, and we're concerned about it too. We've got our eye on the data," Michnick added. Ongoing analyses will expand to include Medicaid populations and more diverse racial and ethnic groups to better reflect the pregnancy population in the United States.