A nationwide French study shows that while nirsevimab provides strong protection against RSV-related hospital admissions during the initial season, its effectiveness diminishes in the second year, prompting questions about ongoing immunity and vaccination strategies.
A nationwide French study published in JAMA Pediatrics suggests that nirsevimab, the monoclonal antibody sold as Beyfortus, remains strongly protective against RSV-related hospital admission during the season it is given, but that benefit does not carry into a second year. The findings also pointed to higher hospitalisation rates for ear, nose and throat bacterial infections and, later, asthma among immunised infants.
Researchers used France’s National Health Data System to compare two matched groups of infants born in metropolitan France before the 2023 and 2024 immunisation campaigns. The analysis included 74,672 infants in the 2023 cohort and 175,526 in the 2024 cohort, with each treated infant matched to an untreated infant by sex, birth month, gestational age, department and social deprivation index.
During the first year of follow-up, RSV lower respiratory tract infection hospitalisations were far less common among infants who received nirsevimab than among those who did not. The effect was consistent across both seasons, despite a shift in the dominant virus strain from RSV A in 2023-24 to RSV B in 2024-25, and despite a sharp increase in uptake of the shot in France.
The protection was strongest in the youngest babies. In the 2024 cohort, infants aged three months or younger had a lower risk than those older than three months, which fits with the drug’s role as a seasonal preventive treatment rather than a long-lasting vaccine.
By the second year of follow-up in the 2023 cohort, however, the advantage had disappeared. Hospital admission for RSV lower respiratory tract infection was the same in both groups, and the study found no meaningful protective association. The authors said that is biologically plausible because nirsevimab provides passive immunity for only about five months, roughly one RSV season.
The study also found an association between nirsevimab exposure and hospitalisation for bacterial ear, nose and throat infections, mainly acute otitis media. That signal was seen in both first-year cohorts and was stronger in the second year. A link with asthma hospitalisation appeared only in the second year. The authors cautioned that these findings are based on relatively small numbers and are difficult to interpret in children under 2, when wheezing illnesses do not always amount to persistent asthma.
For clinicians and pharmacists, the practical message is that nirsevimab’s value remains tied to the RSV season in which it is given. In the United States, the shot is generally used as a single dose before or during the RSV season, and children who enter a second season at higher risk may need a different, higher dose. The French data reinforce that this is temporary protection, not durable immunity.
That matters for families asking whether a baby protected in the first year still needs coverage later. The answer, based on these data and current guidance, is yes: the benefit does not appear to persist into a second RSV season. At the same time, the study does not weaken the main case for nirsevimab, which remains a substantial reduction in RSV-related hospitalisation in infancy.
Disclaimer: This content is for informational purposes only and is not intended to be a substitute for professional medical judgment, advice, diagnosis, or treatment.
