Objective:

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.

Sources:

• JAMA Network Open