Nirsevimab Protects Infants From RSV Hospitalization but the Benefit Fades After One Season
A large French study found strong protection against RSV hospitalization during the first year after nirsevimab, but no measurable protection during the second year.
Nirsevimab substantially reduced RSV-related hospitalizations during the season in which infants received it, with estimated effectiveness of 66% to 72% across two birth cohorts. But the protection did not appear to extend into a second year, and researchers found no evidence that nirsevimab reduced hospitalizations from other infections or later asthma.
Study Details
Nirsevimab, sold as Beyfortus, is a long-acting monoclonal antibody designed to protect infants from respiratory syncytial virus, or RSV. Unlike a traditional vaccine, it provides infants with ready-made antibodies rather than training their immune system to produce its own response.
Researchers wanted to answer an important question: does protection from a single dose extend beyond the first RSV season?
The study also examined whether preventing RSV might reduce other respiratory illnesses or subsequent asthma, an area of interest because severe RSV infection during infancy has been associated with higher rates of asthma later in childhood.
Methodology
Researchers used France’s national health database to create two large matched cohorts of infants born before the country’s RSV immunization campaigns.
The analysis included 37,366 matched pairs of infants in the 2023 cohort and 87,763 pairs in the 2024 cohort. Each infant who received nirsevimab was matched with a similar infant who did not.
Both groups were followed for one year. The 2023 cohort was followed for an additional second year.
Because this was an observational study rather than a randomized clinical trial, the researchers adjusted for differences between the groups, but some residual confounding may remain.
Key Findings
In the 2023 cohort, RSV-related lower respiratory tract infection hospitalization occurred in 0.8% of infants receiving nirsevimab versus 2.4% of unimmunized infants.
In the 2024 cohort, hospitalization occurred in 0.5% versus 1.9%, respectively.
Estimated effectiveness against RSV-related hospitalization was 66% in 2023 and 72% in 2024.
During the second year of follow-up, nirsevimab was not associated with lower RSV hospitalization, with a weighted hazard ratio of 1.03.
Researchers found no clear reduction in hospitalization from non-RSV infections or asthma.
Small increases in some ENT bacterial infection and asthma outcomes were observed, but the researchers cautioned that event numbers were low and said these findings require additional study.
Implications for Practice
The findings reinforce what nirsevimab was designed to do: provide strong passive protection during an infant’s period of greatest vulnerability to severe RSV.
For parents, the important distinction is that nirsevimab should not be viewed as creating years of immunity. Its benefit appears concentrated in the RSV season following administration.
For clinicians, the results provide reassuring real-world evidence across two RSV seasons, including a season with greater RSV B circulation. That is particularly relevant given concerns that changing viral variants could reduce effectiveness.
The absence of second-year protection also supports current strategies that focus nirsevimab use on infants entering their first RSV season and selected higher-risk children entering a second season rather than assuming one dose provides prolonged protection.
The asthma findings deserve caution. Preventing severe RSV may still affect long-term respiratory health, but this study does not establish that nirsevimab prevents asthma. Longer follow-up will be needed.