Real-World Effectiveness of Nirsevimab Against Respiratory Syncytial Virus: A Test-Negative Case-Control Study

IMPORTANCE: Nirsevimab, a long-acting monoclonal antibody, has demonstrated efficacy against RSV-related lower respiratory tract infections (LRTIs) in clinical trials. Post-licensure monitoring is essential to confirm these benefits in real-world settings. OBJECTIVE: To evaluate the real-world 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 used inpatient, outpatient, and emergency room 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. Cases were infants with confirmed RSV infections; controls were those who tested negative. 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 individual risk factors. Separate models examined effectiveness by clinical setting, disease severity, dose, and time since immunization. Broader outcomes, including all-cause LRTI and LRTI-related hospitalization, were also analyzed, with stratification by early and late respiratory seasons. RESULTS: The analytic sample included 3,090 infants (median age 6.7 months, IQR 3.6-9.7), with 680 (22.0%) RSV-positive and 2,410 (78.0%) RSV-negative. 21 (3.1%) RSV-positive and 309 (12.8%) RSV-negative infants received nirsevimab. Effectiveness against RSV infection was 68.4% (95% CI, 50.3%-80.8%). Effectiveness was 61.6% (95% CI, 35.6%-78.6%) for outpatient visits and 80.5% (95% CI, 52.0%-93.5%) for hospitalizations. The highest effectiveness, 84.6% (95% CI, 58.7%-95.6%), was observed against severe RSV outcomes requiring ICU admission or high-flow oxygen. Although effectiveness against RSV infections declined over time, it remained significant at 55% (95% credible interval, 16%-75%) at 14 weeks post-immunization. Protective effectiveness was also observed against all-cause LRTI and LRTI-related hospitalizations during peak RSV season (49.4% [95% CI, 10.7%-72.9%] and 79.1% [95% CI, 27.6%-94.9%], respectively). However, from February to May, when RSV positivity was low, effectiveness against these broader outcomes was negligible. CONCLUSIONS AND RELEVANCE: Nirsevimab provided substantial protection against RSV- related outcomes for at least three months. These findings support the continued use of nirsevimab and provide evidence that may help build public confidence in the immunization program.