Estimated Effectiveness of Nirsevimab Against Respiratory Syncytial Virus

Nirsevimab, a long-acting monoclonal antibody, demonstrated efficacy against respiratory syncytial virus (RSV)-associated lower respiratory tract infections (LRTI) in clinical trials. Postlicensure monitoring is essential to confirm these benefits in clinical settings.

To estimate the effectiveness of nirsevimab against medically attended RSV infections in infants and to assess how effectiveness varies by disease severity, dosage, and time since immunization. DESIGN, SETTING, AND PARTICIPANTS: This test-negative case-control study utilized inpatient, outpatient, and emergency department data from the Yale New Haven Health System. Nirsevimab-eligible infants who were tested for RSV using polymerase chain reaction between October 1, 2023, and May 9, 2024, were included. Infants with RSV-positive results were cases and infants with RSV-negative results were controls. EXPOSURE: Nirsevimab immunization, verified through state immunization registries. MAIN OUTCOMES AND MEASURES: Effectiveness was estimated using multivariable logistic regression, adjusting for age, calendar month, and potential confounders. Separate models examined estimated effectiveness by clinical setting, dosage, time since immunization, and severity (defined as needing high-flow oxygen or intensive care unit admission). Broader outcomes were also analyzed, including all-cause LRTI and all-cause LRTI-associated hospitalization.