National studies had already concluded that this treatment had limited hospitalizations of babies, but the one revealed in The Lancet Child & Adolescent Health provides the most comprehensive overview of the state of knowledge to date. While generally not serious, bronchiolitis, which causes breathing difficulties, particularly in babies during the first six months of life, can sometimes lead to emergency room visits and hospitalizations.
Beyfortus is part of a series of innovative treatments aimed at preventing or limiting infection by the main virus that causes this disease. Nirsevimab, the name of the molecule, is not a vaccine, even though it is injectable, but a preventative treatment that prevents the virus from infecting the body. Following successful clinical trials, this monoclonal antibody—developed by Sanofi in partnership with the British company AstraZeneca—was approved by several regulatory agencies in 2023 and subsequently made available in certain high-income countries.
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83% less risk of hospitalization due to infection
According to the meta-analysis published on May 2, 2025, of 27 studies conducted during the 2023-2024 respiratory syncytial virus (RSV) season in five countries (France, Italy, Luxembourg, Spain, United States), nirsevimab reduces the risk of hospitalization due to infection with the main bronchiolitis virus by an average of 83%, admissions to intensive care by 81%, and cases of lower respiratory tract infections by 75% in children 12 months and younger.
This immunization appeared to be more effective in preventing hospitalization of infants over 3 months old (81%) than those 3 months old or younger (76%), its authors observed.
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Really higher efficiency in the United States?
However, the effectiveness of nirsevimab in reducing bronchiolitis-related hospitalizations appeared to vary across countries, being higher in the United States (93%) than in Spain (83%) and France (76%). A possible explanation, according to the researchers, was a higher proportion of infants at high risk of serious infection who received this treatment in the United States, where this category was prioritized during the 2023-2024 bronchiolitis season due to limited supplies of Beyfortus. Further research would be needed to verify this hypothesis.
This meta-analysis has limitations, its authors acknowledge. The studies included were observational, which may lead to biases related to underlying health problems, socioeconomic status, or regional differences in access to care.
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