Clesrovimab: First Approval.

Clesrovimab (ENFLONSIA™; clesrovimab-cfor) is a long-acting monoclonal antibody developed by Merck & Co., Inc. to prevent respiratory syncytial virus (RSV) disease in infants. It binds a highly conserved epitope at antigenic site IV of the RSV fusion protein, blocking viral entry into host cells and conferring passive immunity. Incorporation of a YTE triple amino acid substitution in the Fc region enhances binding to the neonatal Fc receptor, extending serum half-life and allowing a single, body weight-independent dose. Clesrovimab received its first US approval on 9 June 2025 for preventing RSV lower respiratory tract disease in neonates and infants born during, or entering, their first RSV season. This article summarises the milestones leading to this first approval.