Nirsevimab may be associated with high effectiveness against respiratory syncytial virus disease requiring hospital-based care among infants during the first season of universal publicly funded immunization programs in Canada.
Investigators conducted a test-negative case-control study using prospective surveillance data from 7 tertiary care pediatric hospitals participating in the SPRINT-KIDS Project. The analysis included 1,942 symptomatic infants younger than 12 months who were eligible for nirsevimab and underwent respiratory syncytial virus (RSV) testing between October 27, 2024, and March 1, 2025. Infants whose gestational parent received maternal RSV prefusion F protein vaccination and those who received palivizumab were excluded.
The investigators categorized the infants according to the highest level of care required within 14 days of specimen collection: emergency department (ED) care only, inpatient hospitalization, or intensive care unit (ICU) admission. Nirsevimab exposure was defined as receipt at least 7 days prior to RSV testing. Effectiveness estimates were adjusted for age, sex, epidemiologic week, prematurity, institution, neighborhood income, and high-risk comorbidities.
Among the 1,942 infants, 683 tested positive for RSV and 1,259 tested negative. Overall, 22% (n = 429) of the infants had received nirsevimab. Receipt was less common among those who tested positive.
Adjusted nirsevimab effectiveness against laboratory-confirmed RSV across care settings was 78%. By level of care, estimated effectiveness was 77% against ED visits, 79% against inpatient hospitalization, and 97% against ICU admission.
Protection was observed across clinically relevant subgroups. Estimated effectiveness was 90% among infants born prematurely vs. 76% among those born at term, 86% among infants with high-risk comorbidities vs. 80% among those without comorbidities, and 82% among infants born prior to RSV season vs. 76% among those born during the season.
The data were less conclusive about effectiveness over time. Estimated effectiveness was 85% at 7 to 29 days following nirsevimab administration, 69% at 30 to 59 days, and 78% at 60 to 89 days. The investigators did not report an estimate for 90 to 119 days because of imprecision. The median interval between nirsevimab administration and RSV testing was 41 days, limiting the ability to evaluate longer-term protection.
The investigators noted several limitations. RSV testing was not comprehensive among all symptomatic infants at risk, and testing and hospitalization practices varied across institutions. Nirsevimab receipt could have been misclassified based on caregiver reports, and age measurement was subject to potential error for some infants. De-identified data also prevented determination of multiple episodes among 1 patient. Residual confounding remained possible, and the findings may not be generalizable to other settings.
“Our findings suggest that, coupled with increased uptake, universal nirsevimab programs could play a role in reducing the overall burden of RSV disease,” wrote lead study author Sarah A. Buchan, PhD, of Public Health Ontario, and colleagues.
The study was funded by the Public Health Agency of Full disclosures of the study authors can be found in the study.
Source: JAMA Network Open
