Every fall and every winter, the
bronchiolitis sends thousands of infants to emergency rooms in France, most often after infection with respiratory syncytial virus (RSV). Recently, preventive treatments have become available to limit these serious forms, between vaccines intended for adults and monoclonal antibodies injected into babies, which raises as much hope as questions among parents.
To monitor these new drugs as closely as possible to their actual use, the National Medicines Safety Agency (ANSM) has requested two major surveys in 2024.
pharmacovigilanceas well as a specific French study on the Abrysvo vaccine in pregnant women. Their results, made public at the end of November, scrutinize their tolerance and usage errors identified in the field. With, in passing, a warning on how to administer them.
Bronchiolitis and RSV: what the two surveys show on the safety of preventive treatments
Two categories of preventive treatments against RSV are now available in France. On the one hand, vaccines indicated in adults: Abrysvo (particularly in pregnant women to protect the newborn), Arexvy and mResvia. On the other, a monoclonal antibody, the Beyfortus
(nirsevimab), developed by Sanofi and AstraZeneca, which can only be administered to infants under 24 months of age. The regional pharmacovigilance centers (CRPV) analyzed all the adverse effects declared in 2024 for these products, at the request of the ANSM.
Concerning the Abrysvo, Arexvy and mResvia vaccines in adults, the first report of the 2024/2025 survey “confirms the safety of RSV vaccines Abrysvo and Arexvy used in adults“.”The reports from these two investigations covering the year 2024 make it possible to confirm the safety of these preventive treatments“, underlines the ANSM in a press release. The majority of reported cases are non-serious effects. No new signal calls into question the benefit/risk balance, and monitoring of these vaccines continues for the coming seasons.
Beyfortus, Abrysvo and pregnancy: reassuring data
For Beyfortus, indicated in infants, the 2024/2025 ratio “confirms the safety of Beyfortus“. Here again, many cases analyzed correspond to medication errors : over- or under-dosing, but also confusion with an RSV vaccine reserved for adults. “They highlight a problem of medication errors, particularly between vaccines (Abrysvo/Arexvy) reserved for adults and the monoclonal antibody (Beyfortus) reserved for infants under 24 months.“, adds the ANSM. The agency thus reports the administration in error of the Abrysvo vaccine to three infants, while specifying that “no associated adverse effects“This confusion has not been reported. Apart from these errors, most cases concern RSV infections that occurred despite immunization, which above all reflects that protection is not absolute.
The potential signals suspected in the first 2023/2024 report for Beyfortus (flu syndrome, decreased appetite, decreased muscle tone, risk of stroke in infants) are not confirmed in the new data, even if these events remain monitored. To limit confusion between products, the ANSM has developed a summary sheet for healthcare professionals and reminds that any adverse effects must be reported to the regional CRPV or via the national reporting portal.
In parallel, a large French study EPI-PHARE, carried out using data from the National Health Data System (SNDS) and published in November 2025 in the journal Obstetrics and Gynecologyassessed the safety of the vaccine
Abrysvo when administered during pregnancy. Of nearly 29,000 women vaccinated between September 2024 and January 2025, around 24,900 were compared to unvaccinated pregnant women. Researchers observed no significant increase in the risk of premature birth, stillbirth, small-for-gestational-age, cesarean section, postpartum hemorrhage, preeclampsia, or serious cardiovascular events in mothers.
In women vaccinated before or at 32 weeks of gestation, slight non-significant increases in certain risks were found, in a subgroup which represented only 6% of vaccinated women. These signals therefore remain fragile on a statistical level. In this context, “these results should be interpreted with caution“, underline the authors of the EPI-PHARE study, relayed by the Vidal medical site. They also point out that “this late administration period was chosen following a non-significant increase in premature births observed in women vaccinated before the 32nd week of gestation during the phase III MATISSE trial (study on maternal immunization for safety and effectiveness)“. The results thus support the recommendations of the High Authority of Health, which recommends administering Abrysvo between 32 and 36 weeks of amenorrhea.
Beyond these reassuring French data, the prevention landscape continues to evolve. In September, the European Medicines Agency (EMA) recommended the marketing of another monoclonal antibody, Enflonsia (clesrovimab, MSD laboratory), against RSV in newborns and infants. Each year, during autumn and winter, bronchiolitis affects many babies in France, and these preventive tools are added to hygiene measures and treatment in the event of infection. The authorities point out, however, that, like any medicine, vaccines and monoclonal antibodies can cause undesirable effects and will remain closely monitored over the course of future campaigns.