P-1423. Effectiveness of nirsevimab against RSV-related hospitalizations in a pediatric primary care network using target trial emulation

Abstract Background Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections and hospitalizations among infants. Nirsevimab is a monoclonal antibody to the RSV fusion protein that received U.S. FDA approval in July 2023 and recommended for all children < 8 months old. Real-world studies assessing the effectiveness of nirsevimab are needed.Table 2.RSV hospitalizations, ARTI diagnoses, and antibiotic prescriptions among eligible patients Methods Using target trial emulation, we assessed the effectiveness of nirsevimab across a diverse pediatric primary care network over the 2023-2024 RSV season (October 1 to March 31). To establish a primary care attendee cohort of infants eligible for nirsevimab who remained in the network, we included patients < 8 months old on October 1 with at least one primary care visit within 14 days of birth and at least one primary care visit after turning 8 months old or after the end of RSV season. The primary outcome was an encounter with both a positive RSV laboratory test and hospitalization with an ICD-10 code for RSV bronchiolitis/pneumonia. Secondary outcomes included acute respiratory tract infections (ARTIs) and antibiotic prescriptions for ARTIs during RSV season. Sequential target trial emulation was used and adjusted for follow-up time in monthly intervals for a total of five months.Figure 1.Cumulative incidence of RSV-associated hospitalizations among treated vs. untreated patientsFigure 2.Cumulative incidence of ARTI diagnoses and antibiotic prescriptions among treated vs. untreated patients Results Our study cohort includes 7,208 patients. Among them, 35% of eligible patients received nirsevimab. There were 91 of 4,674 (1.9%) RSV-associated hospitalizations among patients who did not receive nirsevimab and 3 of 2,534 (0.1%) RSV-associated hospitalizations among patients who received nirsevimab. Overall, trial emulation demonstrated that patients who received nirsevimab had a significantly lower rate of RSV-associated hospitalization than those who did not given that the 5-month risk difference for RSV-associated hospitalizations was -0.015 (95% CI: -0.019, -0.001). Children receiving nirsevimab also had fewer ARTI encounters with a 5-month risk difference of -0.038 (95% CI: -0.060, -0.015) and fewer antibiotic prescriptions with a 5-month risk difference of -0.026 (95% CI: -0.040, -0.012). Conclusion Nirsevimab was highly protective against RSV-associated hospitalizations among infants entering their first RSV season and reduced both ARTI encounters and antibiotic use. Disclosures All Authors: No reported disclosures