Study shows nirsevimab was highly effective in preventing severe RSV disease in infants. Credit: Riley Brandt, University of Calgary
Real-World Data Shows Strong Protection From One RSV Shot
In A Nutshell
- A single dose of nirsevimab given before RSV season was tied to an estimated 97 percent drop in ICU admission risk, 79 percent for hospitalization, and 77 percent for emergency room visits.
- Nirsevimab is not a traditional vaccine. It delivers lab-made antibodies directly, giving infants immediate protection instead of prompting their immune system to build one.
- Researchers tracked nearly 2,000 infants across seven Canadian children’s hospitals during the shot’s first season of free, universal availability in Ontario and Quebec.
- Only 22 percent of infants in this hospital-tested sample had received the shot, far below the 69 to 79 percent uptake reported at the provincial level, pointing to room for improved delivery.
A single dose of lab-made antibodies given to infants before their first RSV season was tied to an estimated 97 percent reduction in the risk of needing intensive care, according to new real-world data out of Canada. That same shot was linked to a 79 percent drop in overall hospitalization risk and a 77 percent drop in emergency room visits, numbers that line up closely with what earlier clinical trials found.
Nirsevimab, sold under the brand name Beyfortus, is not a vaccine in the traditional sense. It delivers lab-made antibodies directly into an infant’s system, offering immediate protection against severe RSV disease without waiting for the baby’s own immune system to respond.
Doctors and scientists from seven children’s hospitals in Ontario and Quebec tracked nearly 2,000 infants who came through emergency departments, hospital units, and intensive care units during the 2024-2025 RSV season, the first year the shot was offered to every eligible baby in those provinces for free. Their findings, published in JAMA Network Open, show how much protection one dose can provide, especially for infants who end up needing the most intensive medical care.
Tertiary Hospitals Tracked Nearly 2,000 Infants Tested for RSV
Researchers pulled data from seven children’s hospitals in Ontario and Quebec, part of a Canadian network that tracks infectious diseases in kids. Any infant younger than 12 months who showed up with breathing symptoms and got tested for RSV between late October 2024 and early March 2025 was eligible.
In total, 1,942 infants were included. Their typical age was about 3 months, and just over half were boys. Of that group, 683 tested positive for RSV and were counted as cases, while 1,259 tested negative and served as a comparison group. This method, called a test-negative design, estimates how well a shot works by comparing dosing rates between babies who got sick with RSV and babies who got sick with something else.
Researchers excluded infants whose mothers had gotten a separate RSV vaccine during pregnancy and those who’d received an older RSV prevention drug, since mixing those factors in would have made nirsevimab’s own effect harder to isolate. Calculations were adjusted to account for age, sex, timing within the RSV season, prematurity, income level, and underlying health conditions.
Infants Who Received Nirsevimab Rarely Needed Intensive Care
Only 429 infants, about 22 percent of this hospital-tested group, had received the nirsevimab shot at least seven days before their RSV test. Among babies who tested positive for RSV, just 7 percent had gotten the shot. Among those who tested negative, more than 30 percent had gotten it.
That gap translated into an estimated overall effectiveness of 78 percent against confirmed RSV infection. Broken down by severity, estimated effectiveness was 77 percent against emergency room visits, 79 percent against hospital admission, and 97 percent against needing intensive care, though that top figure came from a small number of ICU cases and carries a wide margin of error, ranging from 85 to 100 percent.
Protection held up across different groups of babies. Among infants born prematurely, effectiveness was estimated at 90 percent. Among babies with existing health conditions that raise their risk, it was 86 percent. Effectiveness was highest, around 85 percent, in the first month after the dose, and stayed strong through the second and third months, though researchers had limited data to track protection much beyond that window, since the shot only became widely available partway through the season being studied.
Uptake Remained Low Despite Free Access to the Shot
Most RSV cases in the study showed up in a tight window between November and January, matching when RSV typically circulates most heavily in Canada.
That hospital-based uptake rate lagged well behind broader provincial figures. Outside this study, Ontario reported roughly 69 percent uptake among infants born in hospital during RSV season, and Quebec reported about 79 percent among infants born during the season, both notably higher than what this tested sample showed. Babies from lower-income neighborhoods and those with existing health conditions were somewhat more likely to have received the shot, suggesting outreach efforts reached at least some of the families who needed it most.
RSV sends a large share of Canadian infants to the hospital every year, and most of those hospitalized are otherwise healthy, not just babies with pre-existing conditions. That is part of why researchers say these findings carry weight beyond the two provinces studied, even in a first-year rollout with inconsistent testing practices still finding its footing.
Researchers pointed to specific ways uptake could improve, including offering the shot more consistently at birth in hospitals, familiarizing clinicians with current recommendations, and improving parent education during pregnancy. None of this means every baby who receives nirsevimab is guaranteed to avoid RSV entirely. But the numbers back a simple idea: a single dose of antibodies, given before RSV season starts, appears to sharply lower the odds that a baby ends up needing hospital-level care for the virus. For parents bracing for a newborn’s first winter, that is a statistic worth understanding correctly.
Disclaimer: This article is based on findings published in JAMA Network Open and is intended for general informational purposes. It is not a substitute for medical advice. Parents with questions about RSV prevention for their infant should consult a pediatrician or healthcare provider.
Paper Notes
Limitations
Researchers noted that RSV testing was not performed consistently for every symptomatic infant, and testing and hospitalization practices varied both within and between hospitals over the course of the season. They also could not identify infants who may have contributed more than one hospital visit to the dataset, since repeat tests taken more than 14 days apart were recorded as separate cases. Some infant ages had to be estimated using the first day of the birth month rather than an exact date, due to privacy rules in certain regions, which could introduce small errors. Because the study only included tertiary care children’s hospitals, which tend to see more severe cases, authors caution the results may not fully apply to community hospitals or other settings. Researchers also had a limited window to observe how protection might fade over longer stretches of time, since the shot only became available partway through the season studied.
Funding and Disclosures
Study was funded by a contract from the Public Health Agency of Canada. Several authors reported receiving grants from the Public Health Agency of Canada during the course of the study. A number of authors also disclosed funding, grants, or personal fees from pharmaceutical or diagnostics companies outside the submitted work, including Merck, Enanta, Roche Diagnostics, AstraZeneca, bioMerieux, DiaSorin, Abbott, Thermo Fisher Scientific, and the Quebec Health Research Fund. Funder had input on study design and reviewed the manuscript but had no role in data collection, analysis, interpretation, or the decision to submit the study for publication.
Publication Details
Study, titled “Respiratory Syncytial Virus–Related Hospitalizations Among Infants Receiving Nirsevimab,” was published in JAMA Network Open (2026;9(9):e2633621, doi:10.1001/jamanetworkopen.2026.33621). It was led by Sarah A. Buchan, PhD, of Public Health Ontario and the University of Toronto, along with co-senior authors Jeffrey C. Kwong, MD, MSc, of the University of Toronto and Stephen B. Freedman, MDCM, MSc, of the University of Calgary, as part of the SPRINT-KIDS Project Team. Full author list includes researchers affiliated with institutions across Ontario, Quebec, Alberta, and British Columbia, Canada.
