France's annual bronchiolitis prevention campaign for babies began on Monday, September 14, bringing with it a new monoclonal antibody called Enflonsia that expands the country's immunisation options against the illness from two to three.
"For the 2026-2027 season, Enflonsia complements the offer of immunisation by monoclonal antibodies," the regional health agency, the Agence Regionale de Sante (ARS), said in a statement. The new treatment joins the existing monoclonal antibody Beyfortus, developed by Sanofi with AstraZeneca, and the vaccine Abrysvo, made by Pfizer and offered to pregnant women.
Bronchiolitis is a common respiratory illness in infants usually caused by respiratory syncytial virus, or RSV, which can inflame the small airways in the lungs and, in severe cases, lead to hospitalisation.
Who Enflonsia is for
Enflonsia is a long-acting preventive monoclonal antibody developed by the American pharmaceutical company MSD, known as Merck Sharp & Dohme. The company says on its website that it is designed to offer direct, rapid and durable protection for five months, corresponding to a typical RSV season.
In France, it is approved for babies during the first year they are exposed to the virus and is given as a single injection, with no dose adjustment based on the child's weight.
Regional health agencies say babies born from September 14, 2026, through the end of the annual immunisation campaign on February 28, 2027, can be offered a dose of Enflonsia directly in the maternity ward a few days after birth, as part of routine maternity care. The dose can also be administered in community healthcare settings. Babies born between February 21, 2026, and September 13, 2026, can receive catch-up immunisation in the community.
Enflonsia can be prescribed by a doctor or a midwife, who can also administer it, as can nurses. Like Beyfortus and Abrysvo, the new treatment is reimbursed at 65% by France's national health insurance system, the Assurance Maladie, with the remaining cost potentially covered by supplementary health insurers known as mutuelles.
How it differs from Beyfortus
Enflonsia is indicated only for a child's first season of exposure to RSV, whereas Beyfortus can also be used during a second season in some children under 24 months who remain vulnerable to severe forms of the illness, according to InfoVac-France, a vaccination information platform, in a question-and-answer document published on its website in late August.
Scientifically, both treatments are long-acting monoclonal antibodies that target the virus's F protein, but they do not target the same antigenic site. Nirsevimab, the active ingredient in Beyfortus, targets antigenic site zero, known as phi, while clesrovimab, the active ingredient in Enflonsia, targets site IV.
InfoVac-France called this difference significant, noting that a mutation allowing the virus to escape one antibody does not necessarily cause resistance to the other. The platform added that mutations conferring resistance to nirsevimab have been observed in France, particularly in RSV type B.
On effectiveness, InfoVac-France said no comparative trial exists to demonstrate that the two treatments are equivalent or that one is not inferior to the other. However, it said efficacy estimates against RSV-related hospitalisations were close in clinical trials, at 83.2% for nirsevimab and 84.2% for clesrovimab.
Approved in the US and UAE since 2025
The European Medicines Agency recommended market authorisation for Enflonsia a year earlier, in September 2025. The treatment had already been approved in the United States and the United Arab Emirates since June 2025.
Asked about stock levels for this first French campaign to include Enflonsia, MSD told the AFP news agency that France had been prioritised for supply of the antibody. The company said it had sufficient volumes to meet the needs of patients and healthcare professionals for the 2026-2027 campaign.
