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The U.S. Food and Drug Administration (FDA) has granted approval to Merck’s new preventive antibody shot, Enflonsia (clesrovimab-cfor), designed to protect infants up to one year of age from respiratory syncytial virus (RSV) during their first RSV season. This landmark decision introduces a major competitor to the existing RSV prophylactic market, currently led by Sanofi and AstraZeneca’s Beyfortus.
Enflonsia is a long-acting monoclonal antibody that provides rapid and durable protection against RSV, which is the leading cause of infant hospitalization in the United States. The approval is based on data from pivotal clinical trials showing that a single dose of Enflonsia reduced RSV-related hospitalizations by over 84% and decreased the incidence of medically attended lower respiratory infections by more than 60% compared to placebo in infants up to five months old. Notably, Enflonsia is the first RSV preventive option for infants that uses the same dose—105 mg—regardless of the infant’s weight, potentially simplifying dosing for healthcare providers.
Merck plans to begin accepting orders for Enflonsia in July, with deliveries expected before the onset of the RSV season, which typically runs from fall through spring. “We are dedicated to ensuring that Enflonsia is accessible in the U.S. before the upcoming RSV season begins, aiming to alleviate the significant impact of this prevalent seasonal virus on families and healthcare systems,” said Dean Y. Li, president of Merck Research Laboratories.
The approval of Enflonsia intensifies competition in the infant RSV market, which has been dominated by Sanofi and AstraZeneca’s Beyfortus. Beyfortus, which requires weight-based dosing, faced supply shortages last year due to unprecedented demand. In response to Merck’s entry, Sanofi has announced ambitious plans to ramp up Beyfortus production, tripling capacity and doubling manufacturing sites since its 2023 launch.
Both Enflonsia and Beyfortus are monoclonal antibodies that offer immediate protection by introducing antibodies directly into the bloodstream. However, they target different aspects of the virus, making direct comparisons challenging. Sanofi highlights Beyfortus’s longer duration of protection (up to six months) and a 75% reduction in medically attended lower respiratory infections, while Merck emphasizes Enflonsia’s single-dose convenience and its potential to reduce resistance development.
Other companies, such as Pfizer, GSK, and Moderna, also produce RSV vaccines, but these are currently approved only for adults or pregnant women. The FDA has paused trials of RSV vaccines for young children while assessing safety issues.
Looking ahead, all companies are awaiting recommendations from the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices, scheduled to meet later this month, which will guide the use of RSV preventives in infants.
Disclaimer:
This news article is based on publicly available information as of June 2025. The details regarding drug efficacy, safety, and availability are subject to change as new data emerges or regulatory guidance is updated. Always consult a healthcare professional for medical advice regarding vaccines and preventive treatments. The author and publisher are not responsible for any actions taken based on the information provided.
