FDA Reviews Safety of RSV Therapies for Infants

U.S. health regulators have informed executives at Merck, Sanofi, and AstraZeneca that their infant respiratory syncytial virus (RSV) preventive therapies will undergo a new safety review. The move comes amid concerns raised by vaccine skeptics and heightened scrutiny from U.S. Health Secretary Robert F. Kennedy Jr., a long-time critic of vaccines who has questioned routine childhood immunizations.

The therapies—Beyfortus from Sanofi and AstraZeneca, and Enflonsia from Merck—provide infants with ready-made antibodies to protect against RSV during the first six months to over a year of life. Unlike vaccines, they do not stimulate the immune system to produce antibodies. These treatments are included in the CDC’s recommended childhood immunization schedule and are designed to reduce hospitalizations from RSV, a virus that can cause severe respiratory illness in infants.

The FDA Inquiry

According to sources familiar with the situation, FDA officials appointed under Kennedy began inquiries into the therapies over the summer. Tracy Beth Hoeg, a senior FDA adviser, raised questions about the safety of the products as early as June. Internal documents show that George Tidmarsh, the former director of the FDA division overseeing RSV therapies, directed staff in late August to review data on Enflonsia following these inquiries. Beyfortus has been available in the U.S. since July 2023.

Last week, the FDA’s Center for Drug Evaluation and Research (CDER) convened a brief call with the three companies to signal that further safety questions would be forthcoming from the commissioner’s office. It remains unclear whether the FDA will change product labels or restrict availability. Andrew Nixon, a Health and Human Services Department spokesman, emphasized that the agency evaluates emerging safety data routinely and updates labeling if necessary, maintaining that decisions are grounded in science.

Responses from Companies and Experts

Sanofi stated that Beyfortus has been evaluated in more than 50 studies involving over 400,000 infants, demonstrating safety and effectiveness. Merck confirmed the FDA meeting and expressed confidence in Enflonsia, welcoming continued scientific dialogue.

RSV therapies are critical in reducing severe illness: annually, two to three out of every 100 infants under six months are hospitalized for RSV, and high-risk infants may experience life-threatening complications. Beyfortus generated global sales exceeding $2.6 billion in 2024, while Enflonsia is projected to bring in $250 million next year. Real-world data indicate up to a 43% reduction in RSV-related hospitalizations for infants during the 2024–25 season compared to pre-pandemic periods.

Some vaccine critics, including journalist Maryanne Demasi, have raised concerns about potential seizure risks, though multiple studies have found no evidence supporting this claim. Infectious disease experts, including Stanford’s Jake Scott, argue that separating data for newborns and older infants is necessary to prevent misattribution of adverse events from other vaccines.

Dr. Robert Malone, a Kennedy-appointed member of the national vaccine advisory board, has publicly questioned his prior support for Enflonsia, reflecting ongoing debates within the advisory system. In the same review session, Hoeg highlighted data from four late-stage clinical trials suggesting a possible imbalance in mortality between treatment and control groups, though the findings were not statistically significant.

The FDA’s review underscores the ongoing tension between precautionary scrutiny and reliance on established scientific evidence. While the inquiry may influence public confidence in RSV therapies, health authorities continue to stress that these preventive treatments play a vital role in protecting vulnerable infants from severe respiratory illnesses.