P-1186. Significant Reduction in Disease Burden and a Shift in Clinical Diagnoses in Children Hospitalized with Respiratory Syncytial Virus (RSV) after Nirsevimab Implementation in Catalonia (Spain)

Abstract Background RSV causes a significant burden including hospitalization in infants. Nirsevimab prophylaxis was implemented in October 2023 in Catalonia (Spain) for all children < 6-months-old to reduce RSV-associated morbidity. We aimed to define the impact of nirsevimab administration on clinical outcomes of hospitalized children with RSV infection. Methods We compared clinical data from RSV-associated hospitalizations in children < 16-years-old between October 2022 - March 2023 and October 2023 - March 2024. Respiratory samples were collected from 11 hospitals of the COPEDICAT network (https://www.copedicat.cat). Laboratory confirmation was performed by a real-time multiplex RT-PCR-based assay (Allplex Respiratory Panel Assay, Seegene, South Korea) or antigen test. Disease severity was defined by the pulmonary score and/or need for PICU care. Results We included 636 hospitalized patients (63.5% in 2022-23 and 36.5% in 2023-24), mostly with lower respiratory tract infections (97.6%). Median age in the 2022-23 season was 4.5 vs 11 months in the following season (p< 0.001). The proportion of children < 6-months-old was also significantly lower in 2023-24 (53.0% vs 31.5%, p< 0.001) (Figure 1). There was a 42.6% reduction in hospitalizations in 2023-24 compared with the previous season; most of the hospitalized children in the 2023-24 season had not been immunized with nirsevimab (72.5%). In 2023-24, there was a higher proportion of bronchitis (40.1% vs 27.2%, p< 0.001) and children treated with bronchodilators and corticosteroids (both p< 0.001) compared to the previous season. The number of PICU admissions decreased by half (60 vs 32), but its percentage in relation to total hospital admissions did not change significantly (14.9% vs 13.8%, p=0.58). No significant differences were found in the clinical score at admission or respiratory support. Conclusion After the introduction of nirsevimab in infants < 6-months-old, children admitted to catalan hospitals were older and the proportion of bronchitis was significantly higher than in the previous season. In addition, there was a reduction of almost 50% RSV-associated hospitalizations and PICU admissions compared to the previous season; most of the hospitalized children had not been immunized with nirsevimab. Disclosures Asuncion Mejias, MD, PhD, MsCS, Astra-Zeneca: Advisor/Consultant|Astra-Zeneca: Honoraria|Enanta: Advisor/Consultant|Janssen: Advisor/Consultant|Janssen: Grant/Research Support|Merck: Advisor/Consultant|Merck: Grant/Research Support|Moderna: Advisor/Consultant|Pfizer: Advisor/Consultant|Pfizer: Honoraria|Sanofi-Pasteur: Advisor/Consultant|Sanofi-Pasteur: Honoraria