Key Takeaways
- Nirsevimab, sold as Beyfortus, was associated with 79% effectiveness against RSV hospitalization in a Canadian study involving 1,942 eligible infant encounters.
- Researchers estimated 97% effectiveness against RSV-related intensive care admission, but that striking estimate came from relatively few ICU cases and was higher than estimates from much of the previous evidence.
- The hospitalization result is more convincing because it broadly agrees with randomized trials and real-world studies conducted in other countries.
- The study provides stronger evidence about protection relatively soon after nirsevimab administration than about protection later in an RSV season.
- Beyfortus is already a major commercial product for Sanofi, but clinical effectiveness alone does not establish how large or durable its future market will be.
Introduction
Nirsevimab, sold as Beyfortus, is a long-acting monoclonal antibody designed to provide infants with passive protection against RSV.
A 97% reduction is the kind of number that can dominate headlines. For investors following Sanofi’s blockbuster RSV antibody Beyfortus, it is also the kind of number that deserves closer inspection.
Researchers studying infants treated at seven Canadian pediatric hospitals estimated that nirsevimab was 77% effective against RSV-related emergency department visits, 79% effective against hospitalization and 97% effective against admission to an intensive care unit.
The study, published September 14 in JAMA Network Open, provides encouraging real-world evidence from Ontario and Quebec after both provinces introduced publicly funded universal infant nirsevimab programs (Buchan et al., 2026).
Nirsevimab, sold as Beyfortus, is a long-acting monoclonal antibody that provides passive protection against respiratory syncytial virus. Unlike a vaccine, it supplies antibodies directly rather than stimulating an infant’s immune system to produce its own antibodies.
The strongest case emerging from the new research does not rest on the eye-catching 97% number.
More persuasive is the finding that nirsevimab was associated with substantially fewer RSV hospitalizations, a result broadly consistent with earlier randomized trials and a growing body of observational evidence.
Important questions nevertheless remain about the precise magnitude of protection against the most severe outcomes, how long protection persists and how evidence from universal programs will translate into the long-term commercial market.
What the Canadian Study Examined
Buchan and colleagues conducted a test-negative case-control study involving 1,942 eligible encounters involving symptomatic infants younger than 12 months who underwent RSV testing at seven tertiary pediatric hospitals in Ontario and Quebec during the 2024-2025 RSV season (Buchan et al., 2026).
Of the 1,942 eligible encounters, 683 involved infants who tested positive for RSV and 1,259 involved infants who tested negative.
The median age was 3 months.
Overall, nirsevimab use was recorded in 429 encounters at least seven days before testing. Only 48 of the 683 RSV-positive cases, or 7.0%, had received it, compared with 381 of the 1,259 RSV-negative controls, or 30.3%.
After adjusting for measured potential confounders, the researchers estimated that nirsevimab was associated with 78% effectiveness against laboratory-confirmed RSV detected in symptomatic encounters requiring emergency-department or inpatient hospital care.
The estimates were 77% for emergency department visits, 79% for hospitalization and 97% for ICU admission (Buchan et al., 2026).
These are relative effectiveness estimates derived from odds ratios. They should not be interpreted as reductions of the same magnitude in an individual baby’s absolute probability of hospitalization or intensive care.
The 97% ICU Estimate Needs Context
The ICU result is impressive, but it is also the result requiring the greatest caution.
There were 81 RSV-positive infants admitted to an ICU. Only three had previously received nirsevimab. The adjusted effectiveness estimate was therefore 97%, with a 95% confidence interval ranging from 85% to 100% (Buchan et al., 2026).
That finding is consistent with protection against very severe RSV, but the Canadian ICU estimate is too imprecise to establish the true magnitude of that protection on its own.
The authors themselves noted that their ICU estimate was higher than previous pooled evidence.
A 2025 systematic review and meta-analysis incorporating 32 real-world studies from France, Italy, Luxembourg, Spain and the United States found that nirsevimab use was associated with substantially lower odds of RSV hospitalization and ICU admission. The pooled odds ratio was 0.17 for hospitalization and 0.19 for ICU admission (Sumsuzzman et al., 2025).
The meta-analysis also found substantial heterogeneity for hospitalization, with an I² of 85.8%, meaning the size of the observed association varied considerably among studies.
The Canadian 97% estimate is therefore better viewed as one particularly favorable result within a larger evidence base rather than a definitive measure of how effectively Beyfortus prevents intensive care admissions.
Randomized Trials Support the Broader Result
The new findings become more persuasive when the hospitalization result is compared with randomized evidence.
In the HARMONIE trial, 8,058 infants in France, Germany and the United Kingdom were randomly assigned to nirsevimab or standard care. RSV-related hospitalization occurred in 11 of 4,037 infants assigned to nirsevimab and 60 of 4,021 receiving standard care.
That translated into 83.2% efficacy against hospitalization (Drysdale et al., 2023).
The absolute numbers provide important perspective. Approximately 0.3% of infants assigned to nirsevimab were hospitalized for RSV compared with 1.5% receiving standard care.
Very severe RSV-associated lower respiratory tract infection occurred in five nirsevimab recipients and 19 infants receiving standard care, producing an efficacy estimate of 75.7% (Drysdale et al., 2023).
An earlier randomized trial, MELODY, also found a substantial reduction in medically attended RSV lower respiratory tract infection.
Its hospitalization endpoint, however, was less conclusive. Hospitalization occurred in six nirsevimab recipients and eight placebo recipients, corresponding to an estimated efficacy of 62.1%. The 95% confidence interval ranged from -8.6% to 86.8%, meaning that particular endpoint did not establish a statistically significant benefit (Hammitt et al., 2022).
The wider evidence therefore tells a more nuanced story than the 97% Canadian estimate alone. Multiple lines of evidence support substantial protection against medically important RSV disease, while estimates for less common severe outcomes are naturally less precise.
Real-World Evidence Is Growing, but Estimates Vary
The Canadian findings also fit into an expanding collection of observational studies conducted since nirsevimab entered routine use.
The 2025 meta-analysis found substantial protection against hospitalization and ICU admission across multiple countries, but effect estimates were not identical from study to study (Sumsuzzman et al., 2025).
That variability is not necessarily evidence that nirsevimab works in one population and fails in another.
Real-world estimates can differ because populations, RSV seasons, healthcare systems, eligibility rules, timing of administration and methods for identifying cases differ. Observational studies can also be affected by differences between infants who receive preventive treatment and those who do not.
Consistency in the overall direction of the evidence is therefore more informative than treating any single effectiveness percentage as universally applicable.
How Long Does Protection Last?
Another important question is how well protection persists over time.
In the Canadian study, the median interval between nirsevimab administration and RSV testing was 41 days (Buchan et al., 2026). This means the study provides stronger evidence about protection relatively soon after administration than it does about effectiveness later in an RSV season.
Estimated effectiveness was 85% at 7 to 29 days after administration, 69% at 30 to 59 days and 78% at 60 to 89 days. The researchers did not provide an effectiveness estimate for days 90 to 119 because statistical uncertainty became too large.
Those numbers should not be interpreted as demonstrating a steady decline. The confidence intervals overlap, and the study was not designed or powered to precisely establish the pattern of waning protection.
Other real-world research has nevertheless raised questions about decreasing effectiveness as more time passes after administration.
This makes duration an important issue for future research, particularly because a preventive antibody intended to protect infants through an RSV season needs to maintain clinically meaningful protection for months rather than weeks.
What the Study Cannot Tell Investors
Beyfortus is no longer a speculative pharmaceutical asset.
Sanofi reported approximately €1.8 billion in Beyfortus sales during 2025, an increase of 9.5%, and said the product was available in more than 45 countries.
For investors, the Canadian results add to evidence that the product can provide clinically meaningful protection under routine conditions rather than only in controlled clinical trials.
That could matter as governments and health systems evaluate whether to introduce or maintain broad infant programs.
Clinical effectiveness, however, does not automatically translate into proportional commercial growth.
The Canadian study did not evaluate pricing, cost-effectiveness, reimbursement, manufacturing capacity, market share or future demand. Nor did it compare the economics of nirsevimab directly with maternal RSV vaccination.
The size of the future Beyfortus market will depend on questions outside the scope of this research, including how many countries adopt broad programs, the prices health systems are willing to pay, competition from alternative RSV prevention strategies and whether protection remains sufficiently strong throughout an RSV season.
The study strengthens the clinical evidence behind the product. It cannot answer those commercial questions.
Limitations
The test-negative design can reduce some biases that affect observational effectiveness studies, but it does not provide the protection against confounding offered by randomization.
Nirsevimab recipients differed from nonrecipients. Premature infants and infants with medical comorbidities, for example, were more likely to have received the antibody. Researchers adjusted for measured differences, but residual confounding remains possible.
Testing and hospitalization practices also differed among hospitals and changed during the season. Exposure could have been misclassified when caregivers incorrectly reported whether an infant had received nirsevimab.
Because the records were deidentified, the researchers could not determine whether some infants contributed more than one illness episode. Some age information was also imprecise because privacy restrictions meant exact birth dates were unavailable in certain jurisdictions.
Finally, the participating institutions were tertiary pediatric hospitals. That gave researchers access to severe RSV cases but may limit how confidently the results can be generalized to infants treated in community hospitals or other healthcare settings (Buchan et al., 2026).
Final Thoughts
The Canadian study is encouraging evidence for nirsevimab, but its most dramatic number should not overshadow its more convincing result.
The 97% estimated effectiveness against ICU admission supports protection against severe RSV, but it comes from relatively few cases and exceeds estimates seen in much of the previous evidence.
The 79% hospitalization estimate is arguably more informative because it fits comfortably within a larger evidence base that includes randomized trials and real-world studies from several countries.
For Sanofi and AstraZeneca, the accumulating clinical evidence strengthens the foundation behind an already commercially successful product.
It does not predict how quickly the market will grow.
Duration of protection, adoption of universal programs, competition from maternal RSV vaccination, pricing and reimbursement decisions will all help determine whether strong clinical effectiveness translates into durable commercial growth.
Evidence from randomized trials and real-world studies increasingly supports that nirsevimab substantially reduces the risk of medically attended and severe RSV disease in infants. The precise magnitude and duration of that protection, and what they ultimately mean for the long-term Beyfortus market, remain important questions.
Funding and Disclosures
The Canadian study was funded through a contract from the Public Health Agency of Canada. The agency provided input on study design and conduct and reviewed the manuscript but, according to the authors, did not collect, manage, analyze or interpret the data and did not make the decision to submit the manuscript.
Several investigators disclosed outside industry relationships, including grants or personal fees involving companies such as Merck, AstraZeneca, Enanta, Roche Diagnostics, bioMérieux, DiaSorin, Abbott and Thermo Fisher Scientific. The study did not report direct funding from the manufacturers of Beyfortus (Buchan et al., 2026).
Industry involvement is more direct in the pivotal randomized evidence. HARMONIE was funded by Sanofi and AstraZeneca, while MELODY was funded by MedImmune/AstraZeneca and Sanofi (Drysdale et al., 2023; Hammitt et al., 2022).
The consistency between manufacturer-funded randomized trials and subsequent real-world evidence from independently funded research strengthens the overall evidence base, although differences in study populations, methods and outcomes mean individual effectiveness estimates should not be treated as interchangeable.
References
Buchan, S. A., Xie, J., Bhatt, M., Papenburg, J., Goldfarb, D. M., Pernica, J. M., Burstein, B., Gravel, J., Berthelot, S., Kam, A. J., Poonai, N., Eltorki, M., Finkelstein, Y., Sabhaney, V., Cheng, J., Gill, P. J., Dickinson, J. A., Kwong, J. C., & Freedman, S. B. (2026). Respiratory syncytial virus-related hospitalizations among infants receiving nirsevimab. JAMA Network Open, 9(9), e2633621. https://doi.org/10.1001/jamanetworkopen.2026.33621
Drysdale, S. B., Cathie, K., Flamein, F., Knuf, M., Collins, A. M., Hill, H. C., Kaiser, F., Cohen, R., Faust, S. N., Kieffer, A., Leach, A., Baca Cots, M., Zar, H. J., Campora, L., & Dagan, R. (2023). Nirsevimab for prevention of hospitalizations due to RSV in infants. The New England Journal of Medicine, 389(26), 2425-2435. https://doi.org/10.1056/NEJMoa2309189
Hammitt, L. L., Dagan, R., Yuan, Y., Baca Cots, M., Bosheva, M., Madhi, S. A., Muller, W. J., Zar, H. J., Brooks, D., Grenham, A., Wählby Hamrén, U., Mankad, V. S., Ren, P., Takas, T., Abram, M. E., Leach, A., Griffin, M. P., & Villafana, T. (2022). Nirsevimab for prevention of RSV in healthy late-preterm and term infants. The New England Journal of Medicine, 386(9), 837-846. https://doi.org/10.1056/NEJMoa2110275
Sumsuzzman, D. M., Wang, Z., Langley, J. M., & Moghadas, S. M. (2025). Real-world effectiveness of nirsevimab against respiratory syncytial virus disease in infants: A systematic review and meta-analysis. The Lancet Child & Adolescent Health, 9(6), 393-403. https://doi.org/10.1016/S2352-4642(25)00093-8
Health Information Disclaimer
This article is for general educational and informational purposes and does not provide individualized medical advice. Recommendations for nirsevimab and maternal RSV vaccination differ by country and may depend on an infant’s age, medical risk, season and the timing of maternal vaccination. Caregivers should consult current local public-health guidance and their child’s clinician when making decisions about RSV prevention.
