US FDA Initiates New Safety Review of Approved RSV Therapies for Infants

U.S. health regulators have informed executives at Merck, Sanofi, and AstraZeneca that approved therapies for preventing respiratory syncytial virus (RSV) in infants will undergo additional safety review. The move comes amid scrutiny from individuals questioning the safety of vaccines and related therapies.

The therapies—Beyfortus from Sanofi and AstraZeneca, and Enflonsia from Merck—provide infants with protective antibodies without triggering the body’s immune system to create its own. These treatments are recommended by the U.S. Centers for Disease Control and Prevention (CDC) as part of the childhood immunization schedule.

FDA officials began raising questions about these therapies over the summer. Tracy Beth Hoeg, a senior FDA adviser, initiated the review in June, citing potential safety concerns. Internal documents show that in late August, the FDA directed staff to compile information on Enflonsia for a second evaluation, while Beyfortus has been available in the U.S. since July 2023.

A recent call between FDA officials and the companies indicated that further questions about safety could be expected, though details about specific data requests were not shared. It is currently unclear whether any regulatory actions, such as label changes or usage restrictions, will be taken. The FDA stated that it routinely reviews safety information and updates product labeling if warranted, emphasizing that its decisions are guided by evidence-based science and patient safety.

Sanofi noted that Beyfortus has been studied in over 50 clinical trials involving more than 400,000 infants, demonstrating safety and effectiveness. Merck confirmed the FDA meeting and expressed confidence in Enflonsia’s safety, highlighting ongoing scientific dialogue with regulators.

RSV can be serious for young infants, with two to three out of every 100 children under six months hospitalized each year. High-risk infants may experience severe respiratory problems, pneumonia, or life-threatening complications. CDC data show that Beyfortus and a maternal RSV vaccine reduced RSV-related hospitalizations by up to 43% in the 2024-25 season compared with earlier periods.

Some critics have raised concerns about potential seizure risks associated with the therapies, though multiple safety studies have not found supporting evidence. Experts emphasize that separating data for newborns and older infants is important to avoid misattributing side effects from other routine vaccines to RSV therapies.

Recent discussions within the FDA and its advisory committees have revisited past recommendations, including the use of RSV therapies and other vaccines for newborns. While some preliminary analyses suggested potential differences in mortality between treated and untreated groups, these findings were not statistically significant and may be due to chance.