180-day efficacy of nirsevimab against hospitalisation for respiratory syncytial virus lower respiratory tract infections in infants (HARMONIE): a randomised, controlled, phase 3b trial
Authors: Alasdair P S Munro, Simon B Drysdale, Katrina Cathie, Florence Flamein, Markus Knuf, Andrea M Collins, Helen C Hill, Friedrich Kaiser, Robert Cohen, Didier Pinquier, Natalya C Vassilouthis, Mariana Carreno, Catherine Moreau, Pierre Bourron, Lydie Marcelon, Karine Mari, Michelle Roberts, Pierre Tissières, Simon Royal, Saul N Faust
Journal: The Lancet Child & Adolescent Health (2025)

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infection and hospitalisations in infants worldwide. The primary analyses of HARMONIE showed that nirsevimab reduced infant hospitalisations due to RSV-associated lower respiratory tract infection through the RSV season. This analysis aims to evaluate nirsevimab's efficacy at 180 days after dosing, a period exceeding the typical 5-month RSV season. HARMONIE is an ongoing, open-label, parallel arm, randomised, controlled, phase 3b study conducted in France, Germany, and the UK. Infants aged 12 months or younger, born at a gestational age of at least 29 weeks, were randomly assigned (1:1) to receive either a single intramuscular dose of nirsevimab (50 mg for children <5 kg or 100 mg for children ≥5 kg) or standard care (without RSV prophylaxis) before or during their first RSV season. Randomisation was electronically done, stratified by country and age-group. The primary efficacy endpoint for this analysis was the incidence of hospitalisations due to RSV-associated lower respiratory tract infection up to 180 days after nirsevimab administration or randomisation in all randomised participants. Safety up to 365 days following nirsevimab administration was also assessed. This trial is ongoing and registered with ClinicalTrials.gov, number NCT05437510. Between Aug 8, 2022, and Feb 28, 2023, 8057 infants were randomly assigned to either the nirsevimab group (n=4038) or the standard care group (n=4019). The median age at randomisation was 4·00 months (IQR 1·0-7·0; range 0·0-12·0, and 4195 (52·1%) were male and 3862 (47·9%) were female. Up to 180 days, 12 (0·3%) of 4038 infants in the nirsevimab group and 68 (1·7%) of 4019 infants in the standard care group had been hospitalised for RSV-associated lower respiratory tract infection, corresponding to a nirsevimab efficacy of 82·7% (95% CI 67·8-91·5; p<0·0001). Most participants experienced grade 1 (2759 [68·7%] of 4016 in the nirsevimab group; 2696 [67·1%] of 4018 in the standard care group) or grade 2 (1447 [36·0%] of 4016 in the nirsevimab group; 1436 [35·7%] of 4018 in the standard care group) treatment-emergent adverse events, and no apparent safety concerns were raised up to 365 days after dosing. Nirsevimab offers consistent and sustained protection against hospitalisation due to RSV-associated lower respiratory tract infection for at least 6 months. This finding provides global health systems greater flexibility when implementing nirsevimab, providing substantial benefit in the ongoing effort to reduce the burden of infant RSV and the potential wider public health value. Sanofi and AstraZeneca.