- Design
- Two nationwide matched cohorts, French National Health Data System
- Population
- About 125,000 nirsevimab-immunised infants matched 1:1 with unimmunised infants
- Primary outcome
- RSV lower respiratory tract infection hospitalisation in years one and two
- Effect
- Year one effectiveness 66% (95% CI 61–70) and 72% (69–75); year two wHR 1.03 (0.73–1.43)
Two French nationwide matched cohorts, from the 2023 and 2024 campaigns, compared infants given a single dose of nirsevimab with matched unimmunised infants — about 125,000 pairs in all.
In the first year, RSV lower respiratory tract infection admissions occurred in 0.8% versus 2.4% (2023) and 0.5% versus 1.9% (2024), effectiveness of 66% and 72%. In the second year of the 2023 cohort there was no protection (wHR 1.03, 95% CI 0.73–1.43).
Two unexpected associations appeared: more admissions for bacterial ear, nose and throat infections in both cohorts (wHR about 1.4), and more asthma admissions in year two (wHR 1.24). These are observational signals without an obvious mechanism and may reflect differences between families who did and did not take up the antibody. They need watching, not a change in practice.
The results contrast with a Galician programme that reported a persisting second-season benefit. For now, parents should be told protection covers the first RSV season.
- Offer nirsevimab where available; first-season protection against RSV admission was 66–72%
- Tell parents protection is for the first RSV season — no second-year benefit was seen
- Do not cite the ENT infection and asthma signals as established harms; they are unexplained associations
- High-risk infants entering a second season need their own protection plan
Why it matters
It settles the counselling question parents ask: whether one dose covers the next winter too.
Don't overread it
The ENT infection and asthma associations are observational and may reflect residual confounding.
The statistics, in plain English
A weighted hazard ratio of 1.03 with an interval from 0.73 to 1.43 means no detectable second-year effect either way. The ENT signal's lower limit in the 2023 cohort was exactly 1.00, so that estimate is borderline.
Read the rest in the app
You have read your two free briefings this month. The app carries all 27 specialties, every morning, free — and this finding is waiting in it.
Scan to keep reading on your phone. No account needed to start.
Tomorrow morning, before your first patient
One edition a day for paediatrics, written by the desk, every claim tied to its paper. Six minutes.
Get the app — free