Multinational retrospective analysis on real-world impact of nirsevimab immunisation and maternal RSV vaccination against respiratory disease

Respiratory syncytial virus (RSV) is a highly common virus that causes infections of the respiratory tract, including the lungs, usually producing mild cold-like symptoms. However, RSV can also cause severe respiratory illnesses such as bronchiolitis and pneumonia, making it especially dangerous for infants and one of the leading causes of hospitalization among young children worldwide. Recently, a major breakthrough in RSV prevention was achieved with the development of nirsevimab, a long-acting monoclonal antibody that provides protection during the period of highest vulnerability in early life.

Nirsevimab was approved for commercialization by the European Medicines Agency (EMA) in 2022, and was pioneeringly introduced in Catalonia (Spain) in October 2023. After the success of the 2023-2024 immunisation campaign in Spain, Luxembourg and other European countries, more have adhered to it in the 2024–2025 season, such as Italy. Nevertheless, coverage, provision and public health policy differed between countries. Alternatively, maternal immunisation with the Respiratory Syncytial Virus prefusion F vaccine (RSVpreF) vaccine was licensed in 2023. The United Kingdom (UK) introduced the RSVpreF in the 2024–2025 season, also with differing coverage and provision within regions.

The aim of the study was to analyse emergency department (ED) attendances and admissions to hospital due to bronchiolitis following RSVpreF and nirsevimab introduction, with hospitals in Iceland and Romania, where no intervention was introduced, used as comparators.

This study is the first multinational European investigation to compare the real-world impact of different RSV prevention strategies on bronchiolitis-related emergency department attendances and hospital admissions in infants. The analysis also allowed the evaluation of the consistency of nirsevimab protection across different seasons and implementation settings. By including hospitals from Iceland and Romania, where no RSV preventive measures had been introduced, the study was able to account for natural year-to-year variations in RSV circulation, providing a more robust assessment of the effectiveness of the interventions.

In the 2024–2025 season, in Catalonia, there was a reduction in the risk ratio (RR, Figure 1) for bronchiolitis ED attendances [0.45 (95% CI 0.43–0.47)] and admissions [0.40 (95% CI 0.37–0.43)] in infants of age <6 months. This translates to an estimated reduction of almost 60% of attended ED bronchiolitis. However, this reduction was not seen in Rome, the other region with nirsevimab immunisation. In two of the sites 

representing the UK in the study, where the RSVpreF vaccine was delivered, there was a significant but modest reduction of hospital admissions due to bronchiolitis for infants of age <6 months (0.80 (95% CI 0.69–0.94) and 0.85 (95% CI 0.76–0.95), respectively). These differences are likely to relate to the reduced uptake of RSV prevention products in these settings compared to Catalonia.

The findings highlight that the success of RSV prevention programmes depends not only on the effectiveness of the preventive products themselves, but also on how they are implemented. The more limited impact observed in the United Kingdom during the first season of maternal RSV immunisation may be related to lower uptake during the initial phase of the campaign, underlining the importance of launching preventive programmes well before the start of the RSV season and promoting public awareness to maximize coverage. Similarly, the absence of a measurable impact in Rome contrasts with the results observed in Catalonia, where uptake rates were higher and eligibility criteria were broader, emphasizing the need for coordinated and well-organised implementation strategies.

Beyond their clinical effectiveness, RSV prevention programmes also raise important questions regarding healthcare planning and resource allocation. The results of this multinational study provide valuable information for countries evaluating different approaches to RSV prevention, but further research is needed to assess their long-term economic impact and cost-effectiveness. Such analyses will be essential for helping health authorities design sustainable prevention policies capable of maximizing health benefits while ensuring the efficient use of healthcare resources.

Reference

The Lancet Regional Health – Europe 2026;63:101597. Published online 3 February 2026. DOI: 10.1016/j.lanepe.2026.101597