Effectiveness of nirsevimab immunoprophylaxis against respiratory syncytial virus-related outcomes in hospital and primary care settings: a retrospective cohort study in infants in Catalonia (Spain)

Background In Catalonia, infants under 6¿months old were eligible to receive nirsevimab, a novel monoclonal antibody against respiratory syncytial virus (RSV). We aimed to analyse nirsevimab’s effectiveness across primary and hospital care outcomes. Methods Retrospective cohort study from 1 October 2023 to 31 January 2024, including all infants born between April and September 2023. We established two cohorts based on nirsevimab administration (immunised and non-immunised). We followed individuals until the earliest moment of an outcome—RSV infection, primary care attended bronchiolitis and pneumonia, hospital emergency visits due to bronchiolitis, hospital admission or intensive care unit (ICU) admission due to RSV bronchiolitis—death or the end of the study. We used the Kaplan-Meier estimator and fitted Cox regression models using a calendar time scale to estimate HRs and their 95% CIs. Results Among 26¿525 infants, a dose of nirsevimab led to an adjusted HR for hospital admission due to RSV bronchiolitis of 0.124 (95% CI: 0.086 to 0.179) and an adjusted HR for ICU admission of 0.099 (95% CI: 0.041 to 0.237). Additionally, the adjusted HRs observed for emergency visits were 0.446 (95% CI: 0.385 to 0.516) and 0.393 (95% CI: 0.203 to 0.758) for viral pneumonia, 0.519 (95% CI: 0.467 to 0.576) for bronchiolitis attended in primary care and 0.311 (95% CI: 0.200 to 0.483) for RSV infection. Conclusion We demonstrated nirsevimab’s effectiveness with reductions of 87.6% and 90.1% in hospital and ICU admissions, respectively. These findings offer crucial guidance for public health authorities in implementing RSV immunisation campaigns.
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