Assessing the Impact of Nirsevimab Immunization on RSV Bronchiolitis Hospital Admissions and Their Severity: A Case-Control Study and Comparison With Pre- and Post-COVID-19 Seasons in a Tertiary Pediatric Hospital.

INTRODUCTION

This study evaluates nirsevimab's real-world effectiveness in preventing respiratory syncytial virus (RSV) bronchiolitis hospitalizations after its introduction in Andalusia's immunization schedule, compares clinical outcomes between immunized and non-immunized infants (2023-2024), and examines RSV seasonality pre- and post-COVID-19.

METHODS

Retrospective single-center case-control study conducted at a tertiary pediatric hospital (Malaga Regional University Hospital). Infants hospitalized for PCR-confirmed RSV bronchiolitis during the first season after nirsevimab introduction were matched with two controls born on the same day. Immunization status, demographic data, and clinical outcomes were analyzed. As a secondary analysis, clinical characteristics and adverse outcomes of immunized and non-immunized RSV bronchiolitis cases from the 2023-2024 season were compared with four previous periods: pre-pandemic decade (04/2010-03/2020), first pandemic season (04/2020-03/2021), second pandemic season (04/2021-03/2022) and third pandemic season (04/2022-03/2023).

RESULTS

Nirsevimab showed 91.5% effectiveness (95% CI: 71.8%-97.4%) in reducing RSV bronchiolitis hospitalizations, which decreased 72.4% from the pre-pandemic average and 82.4% from 04/2022-03/2023. Intensive-care-unit admissions were significantly higher in non-immunized infants (60.0% vs. 26.1%, p = 0.04), with no differences in bacterial superinfection, oxygen requirement and length-of-stay.

CONCLUSIONS

Nirsevimab significantly reduced RSV bronchiolitis hospitalizations during its first season of use. The study also highlights shifts in RSV seasonality trends influenced by the COVID-19 pandemic, highlighting the need for ongoing surveillance to adapt public health strategies.