On June 9, 2025, the U.S. Food and Drug Administration (FDA) approved Merck’s monoclonal antibody, clesrovimab (brand name Enflonsia), for the prevention of respiratory syncytial virus (RSV) in infants during their first RSV season. This decision follows a successful review of clinical data from two pivotal trials: the phase 2b/3 CLEVER trial and the phase 3 SMART trial.
Clesrovimab, a biologic therapy, was shown to significantly reduce the incidence of RSV-related infections and hospitalizations in infants. The approval was based on data from over 3,600 healthy preterm and full-term infants enrolled in the CLEVER trial, who were treated with a single dose of clesrovimab. The trial revealed that a single 105 mg dose of clesrovimab effectively reduced the incidence of RSV-associated lower respiratory infections by 60.5% and RSV-related hospitalizations by 84.3% through five months. Additionally, the drug demonstrated a 90.9% reduction in RSV-related lower respiratory infection hospitalizations and a 91.7% reduction in severe infections, compared to placebo.
The new monoclonal antibody works by targeting a unique site in the RSV protein, which is crucial for neutralizing the virus. Unlike other monoclonal antibodies, such as nirsevimab, which targets a different site, clesrovimab’s action on this specific region makes it highly effective in neutralizing both RSV A and B strains. According to Dr. Octavio Ramilo, a leading infectious disease expert, clesrovimab has demonstrated strong in vitro efficacy against more than 96% of RSV strains, making it a promising new intervention.
Clesrovimab is notable for its extended half-life, meaning that it offers durable protection for up to five months—matching the duration of the typical RSV season. The drug is administered via a single, weight-independent dose, providing a convenient and effective preventive option for infants at risk of RSV.
The approval of clesrovimab represents a major step in the fight against RSV, which remains the leading cause of hospitalization among infants in the U.S. “This is a much-needed preventive treatment,” said Dr. Dean Y. Li, president of Merck Research Laboratories. “We are committed to making Enflonsia available before the upcoming RSV season to help reduce the significant burden of this infection on families and healthcare systems.”
The approval of clesrovimab marks a milestone in advancing treatments for RSV, especially for infants, who are most vulnerable to the disease. With ongoing studies and planned clinical trials, Merck is now looking to expand its use, providing new hope for infants at risk of severe RSV infections.
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