Federal health regulators are taking a second look at antibody treatments designed to protect infants from respiratory syncytial virus, and the reason has less to do with new safety data and more to do with who's asking the questions.

The review represents the latest chapter in a broader reconsideration of routine immunization policies under the current administration. Robert F. Kennedy Jr.'s health leadership team has initiated fresh safety inquiries into RSV preventive therapies, putting approved products from major drugmakers back under the microscope.

On Friday, something notable happened. The CDC's Advisory Committee on Immunization Practices voted 8 to 3 to pull back the universal recommendation for hepatitis B birth-dose vaccines in infants whose mothers test negative for the virus. Instead, the panel recommended individual-based decision-making for parents considering whether to give the hepatitis B vaccine, including the birth dose, to those infants.

For infants not receiving the birth dose, ACIP suggested the initial dose be administered no earlier than two months of age. It's a meaningful departure from longstanding public health guidance.

Drugmakers Get the Call

According to Reuters, senior executives at Sanofi SA (SNY), AstraZeneca Plc (AZN), and Merck & Co. Inc (MRK) were informed that their approved RSV preventive therapies would face additional scrutiny from regulators.

The products in question include Beyfortus, jointly developed by Sanofi and AstraZeneca, and Merck's newer antibody treatment, Enflonsia. Both are monoclonal antibody therapies that provide immediate protection against RSV, unlike traditional vaccines.

Internal documents reviewed by Reuters show that FDA officials appointed under Kennedy began raising questions about these therapies earlier this year. The inquiries weren't sparked by new adverse event reports or clinical findings, but rather by concerns voiced by individuals skeptical of existing vaccine policies.

How the Questions Started

According to Health and Human Services spokesman Andrew Nixon, senior FDA adviser Tracy Beth Hoeg initiated internal safety discussions as early as June. Hoeg has a history of opposing U.S. COVID-19 policies and has publicly questioned certain childhood vaccines.

The scrutiny picked up steam after Australian journalist Maryanne Demasi published a blog post in August suggesting the RSV therapies might increase seizure risk in infants. That claim has been contradicted by multiple safety studies, but it caught attention in certain circles.

By November, George Tidmarsh, who was then director of the FDA division overseeing RSV products, instructed staff to reexamine the approval of Enflonsia, according to documents Reuters obtained. Beyfortus, meanwhile, has been available in the United States since July 2023.

Hoeg's inquiries eventually led the FDA's Center for Drug Evaluation and Research to convene a call with the drugmakers last week, signaling that additional data requests might be coming down the pipeline. The agency hasn't specified exactly what information it plans to seek. Hoeg has since been named acting director of CDER.

Nixon said the FDA is conducting a comprehensive evidence review and will update product labels if deemed necessary.

What the Data Actually Shows

Sanofi told Reuters that Beyfortus has demonstrated safety and effectiveness across more than 50 studies involving over 400,000 infants. That's a substantial body of evidence by any standard.

Unlike vaccines, these antibody therapies deliver immediate protection that lasts for several months to more than a year. They work by providing ready-made antibodies rather than prompting the immune system to produce its own.

A CDC analysis found that Beyfortus and a maternal RSV vaccine reduced infant hospitalizations by up to 43% during the 2024-25 season. That's a meaningful public health impact during peak RSV season, when pediatric hospital beds often fill up quickly.

After Demasi's blog post circulated, Robert Malone, a Kennedy-appointed member of the national vaccine advisory board, said he regretted recommending widespread use of Enflonsia. At a June meeting, Hoeg cited an "unfavorable imbalance" in mortality from late-stage trials, though the differences weren't statistically significant—meaning they could easily have occurred by chance.

The question now is whether the FDA's review will uncover genuine safety signals that previous analyses missed, or whether this represents a more fundamental shift in how regulators evaluate established preventive therapies. The drugmakers are waiting to see what additional data requests arrive, and the broader pharmaceutical industry is watching closely to understand what this means for future product approvals.

For now, both products remain on the market with their current approvals intact, but the additional scrutiny creates uncertainty for companies that have invested heavily in developing RSV preventives for one of the most vulnerable patient populations.