Nirsevimab Effectiveness Against RSV‐Related Hospitalisations in Children Under 24 Months: A Test‐Negative Case–Control Study in Portugal, 2024–2025

ABSTRACT We assessed Nirsevimab effectiveness (NE) against respiratory syncytial virus (RSV)‐related hospitalisation in eligible children (< 2 years) using a test‐negative case–control design within the VigiRSV network (weeks 43/2024 to 16/2025). Among 341 participants (median age: 2 months; 91.2% without known chronic condition), 137 (40.2%) tested RSV‐positive. Adjusted NE against RSV‐related hospitalisation was 78.5% (95%CI: 59.3–89.0). Sensitivity analyses confirmed the robustness of the results. These findings support Nirsevimab's effect in a predominantly healthy infant population and contribute to informing public health decisions for RSV immunisation.