A new immunization, offered free to all infants through universal programs in two Canadian provinces, provided strong protection against respiratory syncytial virus, commonly referred to as RSV, sharply reducing hospitalizations and intensive care unit (ICU) admissions in infants. The findings, published in JAMA Network Open, evaluated the real-world effectiveness of nirsevimab, a long-acting monoclonal antibody that gives infants ready-made antibodies to prevent severe RSV illness. The research was conducted by the University of Calgary–led Surveillance Program for the Rapid identification and tracking of Infectious Diseases in kids (SPRINT-KIDS).
"RSV is a leading cause of hospitalization among infants. We wanted to understand the effectiveness of nirsevimab against severe RSV in infants during their first RSV season," says Dr. Sarah Buchan, PhD, first author on the study and scientific lead, Communicable Disease Control, Public Health Ontario. "The results were dramatic, nirsevimab effectiveness against hospitalizations was 79 per cent, and 97 per cent against ICU admissions."
The study analyzed 2024-2025 RSV season data from seven pediatric hospitals located in Ontario and Quebec.
"One of the most important findings is that most infants with RSV infection were otherwise healthy children. Only three per cent of infants who tested positive for RSV had a comorbidity," says Dr. Stephen Freedman, MD, senior author and SPRINT-KIDS project lead. "This reinforces the value of a universal approach to RSV prevention. Rather than trying to predict which infants will develop severe disease, nirsevimab provides an opportunity to protect all infants during a period when they are particularly vulnerable."
Research indicates up to two per cent of infants born in Canada are hospitalized with RSV each year. Health Canada authorized nirsevimab in 2023 for the prevention of RSV lower respiratory tract disease in newborns and infants entering or during their first RSV season. In the fall of 2024, Ontario and Quebec introduced publicly funded universal programs offering nirsevimab to all infants.
The SPRINT-KIDS Network conducts surveillance activities with funding provided by the Public Health Agency of Canada. The network includes 14 pediatric hospitals across eight provinces and provides real-time surveillance of respiratory viruses and vaccine effectiveness in children. These activities help identify changing patterns of virus activity and provides timely information to support public health policy and clinical care. The information is publicly available here and is included on the Government of Canada's respiratory virus surveillance report.
