Impact of anti-RSV strategies on bronchiolitis across Europe, an interrupted time series analysis

Objectives

To compare the real-life impact of various anti-RSV strategies on bronchiolitis visits to pediatric emergency departments (PEDs) across Europe.

Methods

We performed a quasi-experimental controlled interrupted time-series analysis including all bronchiolitis cases in infants under one year presenting to 28 PEDs in 13 European countries between 01/01/18 and 31/03/25. Five strategies were compared: early nirsevimab with catch-up, late nirsevimab with catch-up, early nirsevimab without catch-up, RSVpreF maternal vaccine, and combined nirsevimab/RSVpreF. Seven centers served as controls. Cumulative changes were pooled using meta-analysis and compared using weighted linear regression, adjusting for coverage. Secondary analyses examined specific age groups, severity, and RSV positivity rate.

Results

A total of 130,562 bronchiolitis visits were included. Impact was highly heterogeneous, ranging from −65% (−81 to − 39) in Luxembourg in 2024–2025 (combined nirsevimab/RSVpreF) to + 11% (−21 to + 56) in London in 2024–2025 (RSVpreF maternal vaccine). The reduction in RSV positivity rate was −44% (−57 to − 30). Adjusted for coverage, the combined nirsevimab/RSVpreF strategy showed the greatest reduction (−42%, −40 to − 32), followed by early nirsevimab with catch-up (−31%, −37 to − 25); the difference was not statistically significant (OR 0.89, 0.76 to 1.01). All other strategies showed significantly lower impact, except for infants under 3 months and hospitalized cases, where no difference was observed across strategies.

Conclusions

Different RSV prevention strategies yield significantly different outcomes, underscoring the importance of implementation framework. This study provides real-world comparative evidence to inform national immunization decisions across Europe.

Key messages

• Impact varied across strategies and settings, ranging from −65% to + 11% change in bronchiolitis visits, highlighting that implementation framework is a critical determinant of outcome.

• The combined nirsevimab/RSVpreF and early nirsevimab with catch-up strategies demonstrated the greatest reductions. Overall, target population and timing of implementation emerged as key drivers.