U.S. health regulators have launched a renewed safety examination of approved respiratory syncytial virus (RSV) preventive treatments for infants, according to sources familiar with the matter. Senior officials at pharmaceutical companies Merck, Sanofi, and AstraZeneca were informed last week that their RSV products would undergo additional scrutiny, despite already receiving regulatory approval.
The therapies under review include Beyfortus, developed jointly by Sanofi and AstraZeneca, and Enflonsia, produced by Merck. The review comes amid heightened skepticism toward vaccines and immunization programs under U.S. Health Secretary Robert F. Kennedy Jr., who has long questioned the safety of vaccines and other preventive treatments.
Kennedy has argued that the potential risks of many pharmaceutical products are not adequately studied. However, major medical organizations and public health officials have warned that revisiting established immunization policies without strong scientific justification could undermine protections that prevent serious illness and save lives.
Internal FDA Questions Trigger Review
According to sources and internal documents, FDA officials appointed during Kennedy’s tenure began raising questions about RSV antibody therapies earlier this summer. Senior FDA adviser Tracy Beth Hoeg reportedly initiated the internal discussions as early as June, though officials have not disclosed what specifically prompted the review.
Hoeg previously criticized public health measures during the COVID-19 pandemic and has questioned the use of certain childhood vaccines. Her inquiries ultimately led FDA officials within the Center for Drug Evaluation and Research (CDER) to hold a brief call last week with executives from all three drugmakers. During the call, company representatives were told to expect further safety-related questions from the commissioner’s office.
While no specific data requests were outlined, sources said the tone of the discussion made clear that regulators are taking the matter seriously. Hoeg has since been named acting director of CDER, further amplifying attention on the review process.
An FDA spokesperson said the agency routinely reassesses safety data for approved products and updates labeling or guidance when warranted. Officials emphasized that the review will rely on evidence-based science and patient safety considerations.
Debate Fueled by Claims From Vaccine Critics
Public debate intensified following claims from critics who suggested the RSV therapies might be linked to seizures. Those claims were circulated in online commentary earlier this year, though multiple safety studies have found no evidence supporting an increased seizure risk.
Former FDA officials overseeing RSV products reportedly instructed staff to reexamine approval data for Enflonsia later in the summer. Beyfortus, which has been available in the U.S. since mid-2023, has already been administered widely.
Sanofi stated that Beyfortus has been evaluated in more than 50 clinical and real-world studies involving over 400,000 infants, demonstrating both safety and effectiveness. Merck confirmed discussions with regulators and said it remains confident in Enflonsia’s safety profile, welcoming continued scientific dialogue.
Why RSV Protection Matters
RSV is a leading cause of hospitalization among infants, with federal estimates showing that two to three out of every 100 babies under six months old require hospital care each year. For high-risk infants, the virus can cause severe respiratory complications, including pneumonia, and can be life-threatening.
Unlike traditional vaccines, RSV antibody therapies provide infants with pre-made antibodies rather than stimulating their immune systems. This approach offers immediate protection during the most vulnerable months of life.
The treatments are included in the CDC’s recommended immunization schedule. Recent real-world data suggest RSV hospitalizations dropped significantly during the most recent respiratory illness season following broader use of these products.
Scientists have warned that limiting access based on unproven concerns could place infants at unnecessary risk and weaken confidence in the scientific review process guiding U.S. public health policy.
