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Merck has received US Food and Drug Administration (FDA) approval for its new infant shot, Enflonsia, aimed at protecting babies from respiratory syncytial virus (RSV)—a common and sometimes severe infection that peaks in incidence during the fall and winter.

The decision sets the stage for Merck to directly compete with a similar product from Sanofi and AstraZeneca, known as Beyfortus.

The newly approved monoclonal antibody treatment is designed for babies under 12 months and will be available in time for the 2025 RSV season. Merck said it expects to begin taking orders in July, with shipments ready before RSV starts spreading widely.

RSV is the leading reason infants are hospitalized, often causing severe lung infections such as pneumonia.

In trials, Enflonsia showed strong results, reducing RSV-related hospitalizations by over 84% and severe respiratory infections requiring medical care by more than 60% compared to a placebo, CNBC reported.

Dr. Dean Li, president of Merck Research Laboratories, said in a statement, "We are committed to ensuring availability of Enflonsia in the US before the start of the upcoming RSV season to help reduce the significant burden of this widespread seasonal infection on families and health care systems."

Enflonsia's key feature is its single, weight-independent dose, making it easier for pediatricians to give and less likely to cause dosing errors.

By contrast, Sanofi's Beyfortus requires different doses based on the baby's weight.

Merck's Enflonsia Adds New Option for Infant RSV Protection

Paula Annunziato, Merck's senior VP of clinical research, said this simplification could "make a big difference for providers and families."

Enflonsia is approved for use during a baby's first RSV season, while Beyfortus can be used for both the first and second seasons in infants who are considered high-risk.

Merck is conducting additional studies to potentially extend Enflonsia's approval for use in infants during their second RSV season.

According to Stat, the price point is also competitive. Enflonsia is listed at $556 per dose, matching Beyfortus.

This FDA approval comes at a time when demand is high and other companies, including Pfizer and Moderna, offer RSV vaccines for pregnant women and adults—but not infants.

The FDA has paused testing RSV shots in younger children due to safety reviews, making these monoclonal antibodies the go-to preventive option for now.

Merck's move adds a new layer of choice and convenience for families and pediatricians. The company hopes the CDC's vaccine advisory panel, scheduled to meet June 25–27, will provide updated recommendations that support Enflonsia's use this season.

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