The UK’s Medicines & Healthcare Products Regulatory Agency has granted marketing authorisation to Enflonsia (clesrovimab), MSD’s monoclonal antibody for preventing respiratory syncytial virus lower respiratory tract disease in neonates and infants during their first RSV season.
Clesrovimab is the first monoclonal antibody designed to protect infants with a single dose regardless of weight, providing coverage for the duration of their first RSV season.
Before the national immunisation programme for infants and older adults was introduced in 2024, RSV was responsible for around 30,000 hospitalisations each year in children under five and 20 to 30 infant deaths.
Adam Finn, Professor Emeritus of Paediatrics at the University of Bristol, said: “There is now very strong evidence from several countries that monoclonal antibody prophylaxis of newborn infants to prevent RSV disease in the first year of life is a highly effective way to prevent winter‑time epidemics and hospital admissions.
He added: “There is substantial benefit in having more than one such product available to help ensure security of supply.”
The authorisation is supported by data from the pivotal phase 2b/3 CLEVER trial and the phase 3 SMART trial, which evaluated clesrovimab in preterm and full‑term infants and compared it with palivizumab in infants at increased risk of severe RSV disease.
In both studies, clesrovimab met its primary endpoint and several secondary endpoints. Results from both trials were recently published in the New England Journal of Medicine.
Across the phase 2b/3 and phase 3 studies, adverse events were reported in 2,854 infants receiving clesrovimab. The most frequent reactions were injection‑site pain, erythema, swelling and rash, with most events mild or moderate.
Ben Lucas, Managing Director UK & Ireland at MSD, said: “We’re delighted with today’s news and the fact that the UK now has another preventative option to tackle the burden of RSV on families and communities. As the NHS continues its shift from sickness to prevention, we encourage UK policymakers to continually review impact of programmes to ensure equitable access to preventable innovations from birth.”
Clesrovimab was approved in the US in June 2025 and received a positive opinion from the EU’s Committee for Medicinal Products for Human Use in September 2025, with further regulatory reviews ongoing worldwide.
