Respiratory Syncytial Virus–Related Hospitalizations Among Infants Receiving Nirsevimab

Importance: Respiratory syncytial virus (RSV) is a leading cause of hospitalization among infants. Nirsevimab, a long-acting monoclonal antibody that protects against severe RSV, was publicly funded for all infants in the Canadian provinces of Ontario and Quebec starting in the fall of 2024. Objective: To estimate the clinical effectiveness of nirsevimab at preventing RSV-related emergency department (ED) visits, inpatient hospitalizations, and intensive care unit (ICU) admissions. Design, Setting, and Participants: This test-negative case-control study obtained data from 7 tertiary care pediatric hospitals located in Ontario and Quebec, Canada, that are members of the Surveillance Program for the Rapid Identification and Tracking of Infectious Diseases in Kids (SPRINT-KIDS) Project. Symptomatic infants younger than 12 months who were tested for RSV in the ED, inpatient units, or ICUs of participating institutions between October 27, 2024, and March 1, 2025, were included. Exposure: Receipt of nirsevimab 7 days or more prior to RSV test date. Main Outcome and Measure: The primary outcome was laboratory-confirmed RSV infection. Participants were stratified by disposition outcome-ED visit only, hospitalization, or ICU admission-on the day of specimen collection or within the subsequent 14 days. Results: Among the 1942 eligible encounters in infants (median [IQR] age, 3.0 [1.9-5.6] months; 1110 males [57.2%]) who were tested for RSV, 683 (35.2%) had RSV-positive test results (cases) and 1259 (64.8%) had RSV-negative test results (controls). Overall, 429 infants (22.1%) received nirsevimab. A lower proportion of cases than controls received nirsevimab (7.0% [48 of 683] vs 30.3% [381 of 1259]; P < .001). Adjusted nirsevimab effectiveness against laboratory-confirmed RSV was estimated to be 78% (95% CI, 68%-84%), including 77% (95% CI, 62%-86%) against ED visits, 79% (95% CI, 66%-87%) against hospitalizations, and 97% (95% CI, 85%-100%) against ICU admissions. Effectiveness remained high across all subgroups, including premature infants (90% [95% CI, 66%-98%]), those with comorbidities (86% [95% CI, 37%-98%]), and time from receipt (85% [95% CI, 75%-92%]); results of sensitivity analyses were consistent with those of primary analysis. Conclusions and Relevance: This test-negative case-control study found that administration of nirsevimab to infants had high estimated effectiveness in preventing RSV disease severity that requires hospital-based care. The results suggest that nirsevimab has the potential to reduce the burden of RSV.