Lea Separovic, Suzana Sabaiduc, Yuping Zhan, Samantha E Kaweski, Sara Carazo, Romy Olsha, Richard Mather, Christine Lacroix, Maan Hasso, Inès Levade, Isabelle Meunier, Agatha N. Jassem, Katie Dover, Ruimin Gao, Nathalie Bastien, DM Skowronski
The Canadian Sentinel Practitioner Surveillance Network used the test-negative design to assess KP.2 vaccine effectiveness (VE) against medically attended outpatient COVID-19 between November 2024 and April 2025 among participants ≥ 12 years. Analyses included 5410 controls (19% vaccinated) and 435 COVID-19 cases (10% vaccinated). Nearly three-quarters (72%) of contributing case viruses were genetically characterized, with XEC (35%) and KP.3.1.1 (38%) variants most commonly identified. Vaccination reduced COVID-19 risk by approximately half (54%; 95% CI: 36-68) relative to unvaccinated individuals. VE was greatest during the first 2 months postvaccination, reducing risk by two-thirds and declining to minimal protection by the fifth month postvaccination.