This form of prophylaxis showed 79% overall effectiveness across 3 European countries.
A multi-country European analysis shows that long-acting monoclonal antibody immunization with Sanofi and AstraZeneca’s nirsevimab (Beyfortus) substantially reduced RSV-related hospitalizations in young children, though protection gradually wanes over several months. Specifically, data from Belgium, Portugal, and Spain show that nirsevimab immunization of children after birth reduced RSV-related hospitalization, with a pooled effectiveness of 79% across the 2,201 children included in the analysis. The study's results were published in Eurosurveillance and examined real-world effectiveness of the long-acting monoclonal antibody nirsevimab during the 2024/25 RSV season.1
Among infants aged 0–6 months who are the newborns at highest risk for severe RSV illness, the overall efficaciousness of immunization with long-acting monoclonal antibodies was 80%.2 "Our results overall and in the age group 0–6 months are in line with efficacy data from clinical trials, observational studies from the 2023/24 season and for the current season from the United States," the investigators wrote.1
RSV immunisation with nirsevimab significantly reduced hospitalizations, showing 79% overall effectiveness across three European countries.
Protection gradually waned over time, decreasing from 85% in the first month to 69% after three months.
Effectiveness remained high in the youngest infants, with 80% protection in babies aged 0–6 months, underscoring the value of early immunisation.
Using data from the 3 aforementioned countries, the investigators conducted a case-control study that included 4,102 hospitalized children under the age of 2. Of these, 791 tested positive for RSV and 1,410 children who tested negative served as controls.1
In the study, children were considered immunized if they received nirsevimab between September 2024 and May 2025 before testing, regardless of the dose, or their age and weight at the time of immunization.1
Since 2022, the European Union has authorized several immunisation products to prevent RSV disease in infants and older adults. These include long-acting monoclonal antibodies administered to newborns during the winter season and vaccines given to pregnant individuals to protect their infants during the first months of life. RSV circulation in Europe has returned to its pre-pandemic pattern, generally spanning October through April.2
Protection declined progressively over time: 85% effectiveness within the first month after immunization, 78% between 30 and 89 days, and 69% between 90 and 215 days. Among infants aged 0–6 months—the group at highest risk for severe outcomes—overall effectiveness remained high at 80%.1
The authors note that monitoring durability of protection across future seasons will be essential. As such a core European Union 2025–26 protocol has been developed based on lessons learned from this pilot study to improve data collection, increase the sample size at the site level and extend the study to more sites for more precise estimates by time since immunization.
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Nirsevimab is a single-dose long-acting monoclonal antibody provided directly to newborns and infants as a single dose, and offers rapid protection to help prevent LRTD caused by RSV, without requiring activation of the immune system.3
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