The Food and Drug Administration (FDA) is investigating the safety of recently approved respiratory syncytial virus (RSV) prevention therapies, including Beyfortus (nirsevimab) and Enflonsia (clesrovimab-cfor). Beyfortus was approved for use in 2023, and Enflonsia this past summer.
The monoclonal antibodies have been recommended for use by the FDA as recently as this summer, but Reuters reports that US Health Secretary Robert F. Kennedy Jr. has questions concerning potential risks of the drugs, including seizures. Several studies have found no signs of increased seizure risks.
According to Reuters, senior FDA adviser Tracy Beth Hoeg, MD, PhD, began raising safety questions internally at the FDA in June. Her efforts led the FDA’s Center for Drug Evaluation and Research to host a call last week with executives from Merck, Sanofi, and AstraZeneca, the maker of the monoclonal antibodies, asking them to provide further safety evidence.
The alleged link to seizures apparently was first made by an Australian journalist this summer in a blog post.
2% to 3% of infants hospitalized for RSV
Kevin Ault, MD, the vice president of the National Foundation for Infectious Diseases (NFID), said the request for more data was inconsistent with prior FDA operations.
If there is post marketing data, then it would and should be reviewed by CDC .
“If there is post marketing data, then it would and should be reviewed by CDC [Centers for Disease Control and Prevention],” Ault told CIDRAP News. “There have been several presentations at ACIP [the CDC’s Advisory Committee on Immunization Practices] on follow-up studies. There are placebo-controlled studies during pregnancy of the RSV vaccine, and there are also placebo-controlled trials of the monoclonal antibodies in infants.”
A representative from Sanofi told Reuters that the safety of Beyfortus has been repeatedly shown in over 50 studies involving more than 400,000 infants.
The antibodies are meant to protect against illness in infants’ first 6 to 12 months of life. RSV is the leading cause of respiratory illness hospitalizations for young infants, with 2% to 3% of all infants hospitalized with RSV each season, according to the CDC.
CDC data show Beyfortus use resulted in a 43% reduction in RSV hospitalizations for infants during the 2024-25 respiratory illness season compared with 2018 through 2020.
CIDRAP News reporter Liz Szabo contributed to this brief.
