Side effects of Beyfortus, the antibody nirsevimab, remain rare and usually mild, according to safety monitoring by the ANSM, the French medicines agency. Its report published on 16 September 2026 describes the safety profile for the 2025-2026 season as reassuring. Above all, it highlights medication errors that persist, notably mix-ups between the baby's antibody and the vaccine given to pregnant women.
Since September 2023, France has offered newborns a monoclonal antibody against respiratory syncytial virus (RSV), the main cause of bronchiolitis. We explain how it works in our article on RSV prevention in infants. Another approach is to vaccinate the mother during pregnancy. The ANSM (Agence nationale de sécurité du médicament et des produits de santé) monitors these products season after season. Here is what its two latest reviews say, and what they mean for parents.
Which Beyfortus side effects have been reported?
The ANSM's first review covers the 2023-2024 season, from 11 September 2023 to 30 April 2024. Over that period, 244,495 doses of Beyfortus were supplied in maternity units and community pharmacies. In total, 198 pharmacovigilance cases were reported.
- The vast majority of these reports, 74.7%, concerned reduced effectiveness or lack of effectiveness: some babies developed bronchiolitis despite the injection.
- These cases of bronchiolitis varied in severity, with no deaths.
- The other reports concerned breathing problems and general effects after the injection: flu-like symptoms, reduced appetite, reduced muscle tone.
- All of these effects resolved well.
A single case of stroke was also reported, without any link to the product being established. Three potential signals were placed under special monitoring. This ANSM page, published on 30 September 2024, was updated on 27 November 2025.
The 2025-2026 review: a safety profile judged reassuring
The report published on 16 September 2026 covers the 2025-2026 prevention season. It was carried out with the regional pharmacovigilance centres in Rouen and Nice. For nirsevimab, more than half of the reports received concern infants who developed RSV bronchiolitis despite the injection. The ANSM specifies that these cases generally had no serious consequences.
A new, recently authorised monoclonal antibody, Enflonsia (clesrovimab), now falls within the scope of this monitoring. Parents may therefore hear about two different antibodies depending on the maternity unit or pharmacy.
Is bronchiolitis despite Beyfortus a side effect?
Strictly speaking, no. It is a limit of effectiveness, which pharmacovigilance also records. No preventive product offers 100% protection. The French studies cited by the ANSM estimate its effectiveness at between 76% and 81% against admissions to intensive care. Another study puts the number of hospital admissions avoided between 15 September 2023 and 31 January 2024 at around 5,800.
Medication errors: the key concern of the 2026 report
The title of the press release sums it up: a reassuring safety profile, but medication errors that persist. The ANSM notably cites:
- giving an RSV vaccine to an infant instead of a monoclonal antibody;
- giving an incorrect dose;
- giving a double dose.
For the vaccines, most of the cases reported are medication errors with no associated adverse effect. The agency is calling on healthcare professionals to stay vigilant. It has published a summary sheet entitled “Bien utiliser les traitements de prévention contre le VRS”.
Mother's vaccine or baby's antibody: not both
The ANSM reminds us that the two approaches should not be combined, except in exceptional cases assessed individually. When the mother has been vaccinated with Abrysvo during pregnancy, the antibody is only recommended for the baby if they are born less than 14 days after the maternal vaccination. The French pregnancy record book now has space to note this vaccination, to avoid giving the child an unnecessary injection.
The background and timing of maternal vaccination are covered in our article on vaccination during pregnancy.
What about the Abrysvo vaccine in pregnant women?
The same report reviews the vaccines, with data from early March 2025 to the end of February 2026. As in the first report, around ten cases of premature birth were reported after vaccination with Abrysvo. No link with the vaccine could be established. A study by Epi-phare carried out in 2025 shows no increase in the risk of premature birth.
Two cases of facial paralysis, which resolved well, were also reported. According to the ANSM, the data currently available are insufficient to establish a link with the vaccine.
What these reviews mean for parents
These data do not change the prevention strategy: the campaign continues, and the recommendations remain those set out in our article on the 2026-2027 bronchiolitis campaign. They do, however, suggest a few simple habits:
- keep a record of what has been given, to both mother and baby, in the pregnancy record book and your baby's health record book;
- tell the healthcare professional about any Abrysvo vaccination during pregnancy before your newborn has an injection;
- don't hesitate to ask for the name of the product at the time of the injection;
- keep an eye on your child over the following days, as after any vaccine on your baby's schedule.
Temporary reactions at the injection site are managed in the same way as for routine vaccines. Our article on vaccine pain in babies explains what helps. If your baby has a fever, the guidance on fever in infants applies: under 3 months, any fever calls for prompt medical advice.
How do you report a side effect?
Parents can report a suspected side effect themselves, without going through a doctor, on the official French portal for reporting adverse health events. These reports feed directly into the ANSM's reviews. Your GP, your paediatrician or a PMI appointment (France's maternal and child health service) can also help you with this.
Key takeaways
- The side effects of Beyfortus reported in France are rare, and usually mild and temporary.
- The ANSM's reviews confirm a positive benefit-risk balance.
- Bronchiolitis is still possible despite the injection, though it is usually not serious.
- The 2026 report focuses on medication errors, notably mix-ups between the antibody and the vaccine.
- If the mother received Abrysvo, the baby only needs the antibody if born less than 14 days after the vaccination.
Sources
- ANSM, “Prévention des bronchiolites à VRS : un profil de sécurité rassurant, des erreurs médicamenteuses qui persistent”, published on 16 September 2026, updated on 17 September 2026.
- ANSM, “Prévention de la bronchiolite du nourrisson : les premières données de pharmacovigilance confirment le profil de sécurité du nirsevimab (Beyfortus)”, published on 30 September 2024, updated on 27 November 2025.
