Nirsevimab cut infant RSV hospitalisations 89% in Quebec's first funded season

When Quebec made the long-acting antibody nirsevimab free to every infant for the 2024-25 respiratory syncytial virus season, it also created the conditions for a clean real-world test of how well the product works outside a company trial. A test-negative study of that programme estimated that nirsevimab was 89% effective against hospitalisation for RSV and 86% effective against an emergency-room visit [s1]. The analysis was funded by the province's own health ministry and public health institute rather than by a manufacturer, which is worth stating up front because most of the effectiveness figures the public has seen for this antibody come from industry-sponsored trials [s1].

RSV is the leading cause of hospitalisation in infants. Nirsevimab is not a vaccine but a monoclonal antibody given as a single injection, providing passive protection for a season. Because it is administered once and confers immunity directly, a programme that reaches most babies can be measured against the babies it missed — the logic behind the test-negative design, which compares antibody receipt among RSV-positive and RSV-negative children who were all tested for the same respiratory symptoms [s1].

What the study measured

Researchers estimated adjusted effectiveness among nirsevimab-eligible children who were tested for RSV during an emergency-room consultation or a hospitalisation between October 2024 and March 2025 [s1]. Eligible infants were either born during the RSV season — the "at-birth" group — or were healthy, preterm, chronically ill or living in remote regions and caught up between roughly six and nineteen months of age [s1].

The effectiveness analysis included 3172 specimens linked to emergency-room consultations, of which 668 (21.1%) were RSV-positive, and 1758 specimens linked to hospitalisations, of which 549 (31.2%) were RSV-positive [s1]. Against emergency consultation, effectiveness was 86% (95% CI 82-90); against hospitalisation, 89% (95% CI 84-92); and against intensive-care admission, 88% (95% CI 58-97) [s1]. Within the hospitalisation estimate, protection was 79% (95% CI 60-89) for children with chronic diseases and 93% (95% CI 83-97) for preterm infants [s1].

Those are high numbers, and they are consistent across the main outcomes. But the width of the intervals matters. The point estimate against ICU admission was 88%, yet its confidence interval ran from 58% to 97% — a spread that reflects how few children reach intensive care, and a reminder that the ICU figure is the least precise of the three [s1]. The chart accompanying this article shows all three estimates with their intervals so the difference in certainty is visible rather than buried.

Turning effectiveness into burden

The authors went beyond effectiveness to estimate how much of the RSV hospitalisation burden a programme like Quebec's could remove. Using 2023-24 respiratory hospitalisation rates, hospital-based RSV surveillance and provincial coverage estimates, they calculated that 41 at-birth immunisations and 58 catch-up immunisations were needed to avert one RSV-associated hospitalisation [s1].

At the coverage the province actually achieved, they estimated that 72% of RSV-associated hospitalisations were prevented in the at-birth group, and 59% in the catch-up group, which had reached 64% coverage by November [s1]. Had coverage instead reached a hypothetical 90% by October — earlier and more complete — the modelled share of hospitalisations prevented rose to 82% [s1]. That last figure is a projection, not an observation, and depends on assumptions about timing and uptake that a single season cannot confirm.

The limits, and the second-season question

A test-negative study controls for many biases but not all of them; families who present a healthy newborn for a seasonal injection may differ from those who do not in ways that inflate an effectiveness estimate, and the design cannot fully rule that out [s1]. The estimates also describe a single province in a single season, when the virus and the population were both behaving in particular ways.

The programme's reach into a second winter is a separate and much less settled question. A recent study of reimmunising high-risk children with a second nirsevimab dose for their second RSV season found only "moderate" protection against hospitalisation, and the estimate was, in the authors' own words, "highly uncertain" — 42% effectiveness with a confidence interval running from -25% to 73%, an interval wide enough to include no benefit at all [s2]. The strong first-season numbers should not be read as a promise about later seasons in older, higher-risk children.

What it means

For a first winter of universal infant coverage, the Quebec data are about as encouraging as real-world effectiveness data get: large, independent of the manufacturer, and consistent across the outcomes that matter most [s1]. The honest caveats are that the most severe-outcome estimate is imprecise, the burden-averted figures include a modelled projection, and protection in a second season is not established [s1][s2]. What to watch next is whether other jurisdictions running their own funded programmes reproduce these numbers, and whether earlier, higher coverage delivers the larger reductions the Quebec model projects.

Sources

• [s1] Nirsevimab effectiveness, number needed to immunize and impact on severe RSV outcomes, a test-negative study in Quebec, Canada. The Lancet Regional Health - Americas, 11 March 2026. https://doi.org/10.1016/j.lana.2026.101448
• [s2] Effectiveness of nirsevimab reimmunization against RSV hospitalization during the second season in high-risk children. Journal of the Pediatric Infectious Diseases Society, 24 September 2026. https://doi.org/10.1093/jpids/piag104