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Nirsevimab (Beyfortus; a long-acting monoclonal antibody administered as a single intramuscular dose) was U.S. FDA-approved for preventing respiratory syncytial virus (RSV) illness in infants and toddlers in July 2023. Industry-sponsored randomized trials have shown efficacy in the range of 70% to 80% (NEJM JW Pediatr Adolesc Med Sep 2020 and N Engl J Med 2020; 383:415; NEJM JW Gen Med Mar 8 2022; [e-pub] and N Engl J Med 2022; 386:837; and NEJM JW Infect Dis Jan 3 2024; [e-pub] and N Engl J Med 2023; 389:2425). Nirsevimab was administered widely in the U.S. during the 2023–2024 RSV season.
To examine real-world effectiveness of nirsevimab, French researchers conducted a prospective case–control study in autumn 2023. Infants younger than 12 …