Association between nirsevimab coverage and pediatric emergency department visits for bronchiolitis in 12 European countries: An interrupted time-series analysis

Background Nirsevimab was introduced to prevent Respiratory Syncytial Virus (RSV) bronchiolitis in infants. Despite its proven effectiveness and similar guidelines across Europe, bronchiolitis reduction varied across Europe. This study explored whether nirsevimab coverage among targeted populations (born in- and out-of-season) was associated to this variability. Methods and findings We used routinely collected data from 27 pediatric emergency departments across 12 European countries (01/2018–03/2024). All bronchiolitis cases aged <12 months were included. Regions were classified as intervention or control based on nirsevimab implementation. Coverage was obtained from official regional reports. We estimated changes in monthly case using controlled interrupted time-series models accounting for seasonality and temporal trends, by age-groups (0–3 and 3–12 months) and regions during the intervention period (10/2023–03/2024) and examined their correlation to coverage using Pearson’s test. Urinary tract infections served as a control outcome. We included 107,088 bronchiolitis cases. Reductions varied widely across regions and age-groups, from +0.6% (95% CI [−9.4, +20.5]) to −61.0% (95% CI [−67.8, −35.2]). Coverage strongly correlated with bronchiolitis reduction across age-groups and regions ( ρ = −0.86, p = 0.001). Bronchiolitis trends in control countries remained stable, as did urinary tract infections. As an ecological study, causality cannot be inferred and unmeasured regional differences may have contributed to the variability. Conclusions This multinational interrupted time-series analysis identified coverage as a key factor associated to bronchiolitis reduction across age-groups and European regions. Implementation choices across Europe likely influenced coverage, highlighting that nirsevimab benefits depend not only on its intrinsic efficacy, but also on how effectively it is delivered.