FDA Opens Safety Review of RSV Monoclonal Antibodies Used in Infants

US regulators have begun a new safety inquiry into the 2 monoclonal antibody (mAb) products currently authorized to protect infants from respiratory syncytial virus (RSV), according to reporting by Reuters. Senior executives at Sanofi, AstraZeneca, and Merck were notified last week that their products, Beyfortus (nirsevimab) from Sanofi and AstraZeneca, and Enflonsia (pavilizumab) from Merck, will undergo fresh scrutiny by the FDA. The agency has not indicated whether the review may ultimately lead to changes in labeling, availability, or postmarketing requirements, and has not yet specified what additional data it will request from manufacturers.

A spokesperson for the Department of Health and Human Services (HHS), Andrew Nixon, told Reuters that the FDA is “rigorously reviewing the available data” and will “update product labeling if warranted by the totality of the evidence.” The review, he said, reflects the agency’s standard approach to emerging safety signals.

Why the Review and Why Now?

According to Reuters, internal agency communications show that questions about the RSV antibodies first surfaced within the FDA over the summer. Tracy Beth Høeg, then a senior adviser within the agency, began asking staff to evaluate possible complications reported in infants who received the products. Høeg has since been named acting director of the FDA’s Center for Drug Evaluation and Research (CDER), the division responsible for oversight of prescription therapeutics.

Her inquiries prompted George Tidmarsh, then head of the FDA division overseeing RSV products, to request a second look at Merck's pavilizumab June approval, Reuters reported.Nirsevimab received FDA approval earlier, in 2024. In late August, CDER staff were directed to compile internal information on pavilizumab in advance of potential follow-up review.

Last week, FDA officials held a short call with executives from the 3 drugmakers informing them to expect further safety questions from the commissioner’s office, Reuters reported. No specific requests for data were made during that call.

Concerns Raised by Outside Commentators

It appears that some of the renewed attention to RSV antibody safety stems from critiques circulated by individuals outside conventional academic and regulatory circles. Reuters noted that an August blog post by Australian journalist Maryanne Demasi, a previous critic of COVID-19 vaccines, suggested that RSV antibodies might increase seizure risk. Several safety studies have not found evidence supporting such an association, according to the reporting.

Separately, internal FDA discussions cited by Reuters indicate that Høeg raised concerns about mortality imbalances observed across 4 late-stage clinical trials of the 2 mAbs. These imbalances were not statistically significant and could represent chance variation, but Høeg suggested the issue “could be revisited” by advisory committees.

Some infectious disease specialists have pushed back on critiques of the safety database. Stanford infectious-disease physician Jake Scott, MD wrote in Quillette, an online magazine, that combining newborn and older infant data when examining seizure rates may misattribute events, especially because older infants receive multiple vaccines concurrently, whereas newborns typically do not. “Rolling back RSV availability based on baseless concerns would do harm to American children,” Scott wrote, arguing that restricting the products without strong evidence would undermine scientific advisory processes. In response, HHS spokesperson Nixon told Reuters that the concerns “are not baseless,” emphasizing that FDA must evaluate all available evidence.

Clinical Role of RSV Antibodies

Nirsevimab and pavilizumab are mAbs that offer passive immunity during early infancy when RSV hospitalization risk is highest. Unlike vaccines, these products deliver ready-made antibodies and do not stimulate the infant’s immune system. Federal health estimates suggest that 2–3 of every 100 infants under 6 months are hospitalized with RSV each year, and severe cases can lead to pneumonia and respiratory distress.²

Nirsevimab has accumulated extensive real-world experience since its US launch. Sanofi told Reuters that the product’s safety and efficacy have been demonstrated in “over 50 studies involving more than 400,000 infants.” Merck and AstraZeneca did not comment on the inquiry.

Recent data from the CDC, cited by Reuters, show that nirsevimab and a maternal RSV vaccine together reduced infant RSV hospitalizations by up to 43% during the 2024–25 respiratory season compared to pre-rollout years.

What's Next

For now, the FDA has not signaled changes to product availability or guideline placement. Both mAbs remain recommended in the CDC’s immunization schedule and continue to be used widely in routine pediatric care. The agency’s review appears to be in its early stages, and neither FDA nor HHS officials have provided a timeline for next steps.

“FDA routinely evaluates emerging safety information and will update product labeling if warranted,” Nixon told Reuters, reiterating that decisions “remain rooted in evidence-based science and in the best interest of patients.”

Clinicians caring for infants at risk for severe RSV should continue to follow CDC and American Academy of Pediatrics guidance while monitoring for FDA updates. As more information becomes available, the agency is expected to clarify whether any additional review, labeling changes, or safety communications will be necessary.

1. Levine D, Wingrove P. Exclusive: FDA launches fresh scrutiny of approved RSV therapies for infants. Reuters. December 9, 2025. Accessed December 10, 2025. https://www.reuters.com/business/healthcare-pharmaceuticals/us-fda-launches-fresh-safety-scrutiny-approved-rsv-therapies-infants-2025-12-09/

2. Clinical overview of RSV. Centers for Disease Control and Prevention. Reviewed August 18, 2025. Accessed December 10, 2025. https://www.cdc.gov/rsv/hcp/clinical-overview/index.html