[{"id":"mel-ep1","name":"Medically-attended RSV-LRTI","type":"PRIMARY","unit":"% reduction","p_value":"<0.001","results":[{"ci":"49.6-87.1","arm":"mel-nirs","label":"74.5% reduction vs placebo","value":74.5}],"description":"Reduction in RSV lower respiratory tract infections requiring medical attention. A single shot before RSV season protects infants for ~5 months - like a vaccine but delivered as a long-acting antibody."}]

Phase 3, randomized, double-blind, placebo-controlled (N=1,490; 994 nirsevimab, 496 placebo). Single IM injection. PRIMARY: Medically attended RSV-associated LRTI through 150 days: nirsevimab 1.2% vs placebo 5.0%. Efficacy: 74.5% (95% CI: 49.6-87.1; p<0.001). RSV hospitalization: nirsevimab 0.6% vs placebo 1.6%. Hospitalization efficacy: 62.1% (95% CI: -8.6 to 86.8; NS). Pooled with Phase 2b: RSV hospitalization efficacy 77.3% (95% CI: 50.3-89.7; p<0.001). Published NEJM March 2022.

N=987 nirsevimab, N=491 placebo. SAEs: 6.8% vs 7.3% (comparable). Most common adverse reactions: rash 0.9% vs 0.6%, injection site reactions 0.3% vs 0%. 97% of reactions mild-moderate. Treatment-related AEs: 1.3% vs 1.5% through 360 days. Deaths: 0. Anti-drug antibodies at Day 361: 5.0%; neutralizing in 21% of ADA-positive. Post-marketing: rare hypersensitivity (urticaria, dyspnea, cyanosis).

SOBI receives 30-35% royalties on Beyfortus US sales from Sanofi. Q4 2025 royalty: SEK 849M. Key debate: Enflonsia (Merck) approved June 2025 as first direct competitor. Nirsevimab uptake approximately doubled year-over-year (VFC doses: 46,738 to 102,057). Real-world effectiveness 77-90% across seasons.

Long-acting anti-RSV monoclonal antibody with ~150-day half-life. Single IM injection provides passive immunity for entire RSV season. Replaces palivizumab (Synagis) which required monthly injections and was limited to high-risk preterm infants.