To assess the effectiveness of nirsevimab against respiratory syncytial virus (RSV) disease requiring hospital-based care among infants during the first season of universal publicly funded immunization programs in Canada.
Approach:
Study Design: A test-negative case-control study using prospective surveillance data from 7 tertiary care pediatric hospitals participating in the SPRINT-KIDS Project.
Participants: Included 1,942 symptomatic infants younger than 12 months who underwent RSV testing between October 27, 2024, and March 1, 2025.
Exclusions: Infants whose gestational parent received maternal RSV vaccination or those who received palivizumab were excluded.
Effectiveness Measurement: Nirsevimab exposure was defined as receipt at least 7 days prior to RSV testing, with effectiveness adjusted for various factors.
Key Findings:
Overall adjusted nirsevimab effectiveness against laboratory-confirmed RSV was 78%.
Effectiveness was 77% against ED visits, 79% against inpatient hospitalization, and 97% against ICU admission.
Effectiveness estimates varied by subgroup: 90% among premature infants, 86% among those with high-risk comorbidities.
Effectiveness decreased over time: 85% at 7-29 days, 69% at 30-59 days, and 78% at 60-89 days post-administration.
Limitations:
RSV testing was not comprehensive among all symptomatic infants at risk.
Testing and hospitalization practices varied across institutions.
Nirsevimab receipt could have been misclassified based on caregiver reports.
Potential errors in age measurement for some infants.
De-identified data limited the ability to assess multiple episodes in one patient.
Residual confounding may affect results, and findings may not be generalizable.
