Background Several clinical trials have shown that nirsevimab, an antibody targeting the respiratory syncytial virus (RSV), reduces RSV-bronchiolitis requiring admission. In 2023-2024, Catalonia and Andorra adopted immunization strategies for children < 6 months and those born during the epidemic season. This study evaluates the effectiveness of nirsevimab in preventing hospitalizations from RSV bronchiolitis. Methods In the epidemic season of 2023-2024, a test-negative case-control study was conducted in three hospitals from Catalonia and Andorra. Patients <12 months old admitted with bronchiolitis and tested for RSV using molecular microbiology tests were included. The effectiveness in preventing RSV-bronchiolitis hospitalization and severe disease was estimated using multivariate models. Comparisons between immunized, non-immunized and non-eligible patients were made in prospectively collected epidemiological, clinical, and microbiological variables. Results 234 patients were included. RSV was detected in 141/234(60.2%), being less common in the immunized group (37% vs 75%, p<0.001). The rate of immunized patients among those eligible was 59.7%. The estimated effectiveness for RSV-associated lower respiratory tract infection was 81.0% (95% confidence interval: 60.9-90.7), and for preventing severe disease (the need for NIV/CMV), 85.6% (41.7-96.4%). No significant differences by immunization status were observed in patients with RSV concerning viral co-infections, the need for NIV/CMV or length of hospital stay. Conclusions This study provides real-world evidence of the effectiveness of nirsevimab in preventing RSV-LRTI hospitalization and severe disease in infants during their first RSV season following a systematic immunization program. Immunized patients did not exhibit a higher rate of viral co-infections nor differences in clinical severity once admitted.

Introduction Rhinoviruses (RV) and Enteroviruses (EV) are among the main causative etiologies of Lower Respiratory Tract Infection (LRTI) in children. The clinical spectrum of RV/EV infection is wide, which could be explained by diverse environmental, pathogen-, and host-related factors. Little is known about the nasopharyngeal microbiota as a risk factor or disease modifier for RV/EV infection in pediatric patients. This study describes distinct nasopharyngeal microbiota profiles according to RV/EV LRTI status in children. Methods Cross-sectional case-control study, conducted at Hospital Sant de Déu (Barcelona, Spain) from 2017 to 2020. Three groups of children <5 years were included: healthy controls without viral detection (Group A), mild or asymptomatic controls with RV/EV infection (Group B), and cases with severe RV/EV infection admitted to the pediatric intensive care unit (PICU) (Group C). Nasopharyngeal samples were collected from participants for viral DNA/RNA detection by multiplex-PCR and bacterial microbiota characterization by 16S rRNA gene sequencing. Results A total of 104 subjects were recruited (A=17, B=34, C=53). Children’s nasopharyngeal microbiota composition varied according to their RV/EV infection status. Richness and diversity were decreased among children with severe infection. Nasopharyngeal microbiota profiles enriched in genus Dolosigranulum were related to respiratory health, while genus Haemophiluswas specifically predominant in children with severe RV/EV LRTI. Children with mild or asymptomatic RV/EV infection showed an intermediate profile. Conclusions These results suggest a close relationship between the nasopharyngeal microbiota and different clinical presentations of RV/EV infection.