Merck's Enflonsia RSV Shot Gains FDA Approval, Competing with Sanofi-AstraZeneca

On June 9, 2025, the U.S. Food and Drug Administration (FDA) granted approval for Merck's monoclonal antibody treatment, Enflonsia, aimed at protecting infants from respiratory syncytial virus (RSV). This decision positions Merck to launch its product ahead of the upcoming RSV season, which typically begins in the fall and lasts through spring. With RSV causing significant morbidity and mortality, particularly among infants and the elderly, the introduction of Enflonsia is expected to provide a crucial new option for healthcare providers and families.

The approval was announced after a comprehensive evaluation of clinical trial data, which indicated that Enflonsia effectively reduced RSV-related hospitalizations by over 84% and hospitalizations due to lower respiratory infections by 90% compared to a placebo among infants aged five months and younger. These findings were detailed in a study conducted by researchers at Merck Research Laboratories, led by Dr. Dean Li, the president of the division, who emphasized the company’s commitment to ensuring the availability of Enflonsia before the RSV season starts.

This new treatment will directly compete with Beyfortus, a similar monoclonal antibody developed by Sanofi and AstraZeneca, which has been a market leader but faced supply shortages during the last RSV season due to unprecedented demand. According to a statement from Sanofi, efforts are underway to ramp up production and ensure an adequate supply of Beyfortus, which recorded sales of approximately €1.7 billion ($1.8 billion) last year.

Monoclonal antibody treatments like Enflonsia and Beyfortus work by delivering antibodies directly into the bloodstream, offering immediate protection against RSV. However, they target different aspects of the virus, which complicates direct comparisons. Notably, Merck's treatment can be administered irrespective of an infant's weight, providing potentially greater convenience compared to Beyfortus, which requires dosage adjustments based on weight.

In the current landscape, several companies, including Pfizer, GSK, and Moderna, are also developing vaccines for RSV, primarily targeting adults and pregnant women. However, the FDA has recently paused trials of RSV vaccines in young children to assess safety concerns, further complicating the market dynamics.

The approval of Enflonsia comes at a critical moment as healthcare professionals await recommendations from a meeting of the Centers for Disease Control and Prevention (CDC) advisors, scheduled for June 25-27, regarding RSV shots and other immunizations. As the medical community prepares for the upcoming RSV season, the introduction of Merck's Enflonsia is anticipated to alleviate some of the burden placed on healthcare systems due to RSV-related hospitalizations.

In conclusion, the entry of Merck's Enflonsia into the market signifies an important development in the fight against RSV, providing healthcare providers with an additional tool to combat this prevalent virus. As the RSV season approaches, the availability of both Enflonsia and Beyfortus will be crucial in addressing the health challenges posed by RSV, particularly for vulnerable populations such as infants and the elderly. The competition between these treatments may ultimately lead to improved outcomes for patients and families affected by RSV-related illnesses.