Nirsevimab linked to 78% less lab-confirmed RSV in a Canadian infant study

A long-acting antibody given to protect infants through their first respiratory syncytial virus (RSV) season was associated with markedly less severe disease in a real-world Canadian study, though the design means the number is an estimate of association rather than proof from a trial [s1]. The study, published in JAMA Network Open, looked at how children fared after Ontario and Quebec began offering nirsevimab to all infants in the autumn of 2024 [s1].

Nirsevimab is not a vaccine. It is a monoclonal antibody — marketed as Beyfortus and licensed to AstraZeneca and Sanofi, according to the US regulator's approval record — that supplies ready-made protection by a single intramuscular injection rather than teaching the immune system to make its own [s2]. That distinction matters for infants, whose own immune response is slow and who face the highest RSV risk in the first months of life.

What the study did

Researchers used a test-negative case-control design, a method built for exactly this question. Symptomatic infants younger than 12 months who were tested for RSV at seven tertiary paediatric hospitals in Ontario and Quebec between 27 October 2024 and 1 March 2025 were included; those who tested positive were the cases and those who tested negative the controls, and the analysis compared how many in each group had received nirsevimab [s1]. Because both groups sought care with respiratory symptoms, the design helps strip out differences in health-seeking behaviour that plague simpler comparisons.

Among 1,942 eligible encounters (median age 3.0 months; 1,110, or 57.2%, were boys), 683 (35.2%) tested positive for RSV and 1,259 (64.8%) tested negative [s1]. Overall, 429 infants (22.1%) had received the antibody [s1]. Exposure was counted only when nirsevimab had been given at least seven days before the RSV test [s1].

What it found

The headline contrast is stark: 7.0% of cases (48 of 683) had received nirsevimab, against 30.3% of controls (381 of 1,259) [s1]. After adjustment, the estimated effectiveness against laboratory-confirmed RSV was 78% (95% confidence interval, 68% to 84%) [s1].

Protection rose with severity. The antibody was linked to a 77% lower chance of an RSV-related emergency department visit (95% CI, 62% to 86%), 79% against hospitalisation (66% to 87%), and 97% against admission to intensive care (85% to 100%) [s1]. Effectiveness held up across subgroups the authors examined, including premature infants at 90% (95% CI, 66% to 98%), infants with other medical conditions at 86% (37% to 98%), and across time since the injection at 85% (75% to 92%) [s1]. Sensitivity analyses were consistent with the main result [s1].

How to read it

Two features make this estimate more credible than a typical observational number. First, unlike the pivotal trials that won nirsevimab its licence, this was not run or funded by the manufacturer: it describes the rollout of a publicly funded programme, and no industry funder is listed [s1]. Second, the test-negative design and the consistency across severities and subgroups all point the same way.

The cautions are the ordinary ones for this kind of study. A case-control estimate cannot rule out residual confounding — families who accept a new antibody for their baby may differ in ways that also lower RSV risk — and the very wide confidence interval on the comorbidity subgroup (37% to 98%) is a reminder that some of these figures rest on small numbers [s1]. The 97% against intensive care is striking but its interval reaches down to 85%, and the ICU group is the smallest [s1]. The study also covers a single season in two provinces, so how well it generalises to other settings, later seasons, or a full season's worth of exposure is not settled here [s1].

The design also captures only children who were sick enough to be tested. That is what makes the test-negative method work — cases and controls are drawn from the same care-seeking pool — but it means the estimate describes protection against disease serious enough to bring an infant to hospital, which is exactly the outcome that matters most, rather than protection against every mild RSV infection [s1]. The figures here sit in the same range as the manufacturer-run licensing trials, and the fact that an independent, publicly funded rollout reproduced them is itself part of the story: effectiveness seen in a controlled trial does not always survive the messier conditions of a national programme, and here it broadly did [s1].

What to watch

The practical questions now are about durability and reach: whether a single dose holds across the whole season, how effectiveness looks once more seasons are pooled, and whether uptake — 22.1% of tested infants in this sample — climbs as the programmes mature [s1]. Real-world monitoring of this kind, repeated season after season, is what will show whether the trial-era promise of RSV antibodies survives contact with routine practice.

This article describes research and is not medical advice. Decisions about infant RSV prevention are for families and their clinicians.

Sources

• Respiratory Syncytial Virus-Related Hospitalizations Among Infants Receiving Nirsevimab — JAMA Network Open, 14 September 2026
• Drugs@FDA: Beyfortus (nirsevimab-alip), BLA 761328 — manufacturer and approval record — U.S. Food and Drug Administration