Expert: Monoclonal antibody targeting the prefusion conformation of the RSV F (fusion) glycoprotein. Prevents viral attachment and membrane fusion with respiratory epithelial cells. Extended half-life achieved through Fc modifications (YTE mutations in nirsevimab; similar approach in clesrovimab) enabling single-dose seasonal protection.

Everyday: RSV (respiratory syncytial virus) uses a protein on its surface called F protein to fuse with and enter human lung cells. These antibodies stick to the F protein like a cap, blocking the virus from getting inside cells. A single injection provides protection for an entire RSV season.

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Merck anti-RSV mAb. FDA approved June 2025. First direct competitor to Beyfortus. Key advantage: single fixed-dose prefilled syringe (vs Beyfortus weight-based dosing). 84% reduction in RSV hospitalization. Key threat to Beyfortus market share. Simpler dosing could drive formulary preference.