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We evaluated 2024/25 KP.2 vaccine effectiveness (VE) against COVID-19 hospitalization among adults ≥60 years old eligible for publicly-funded vaccination during fall and/or spring campaigns in the province of Quebec, Canada. We included Quebec residents tested for COVID-19-compatible symptoms in an acute-care hospital between October 13, 2024 (epi-week 2024-42) and August 23, 2025 (2025-34), linking vaccine, hospital, chronic diseases and laboratory administrative records to assess VE through test-negative design. We compared the odds of being COVID-19 test-positive versus test-negative among vaccinated versus non-vaccinated participants, adjusting for sex, age, comorbidities, place of residence, and epidemiological week. Overall, 49,949 (43%) participants were vaccinated. Over an analysis period spanning up to ten months, including median time since vaccination of 16 weeks (interquartile range 9-24 weeks), VE was 34% overall, declining from 43% <8 weeks to negligible by the 32nd week post-vaccination. Findings confirm meaningful but short-lived COVID-19 vaccine protection against hospitalization in older adults.

Highlights

• We estimate 2024/25 KP.2 vaccine effectiveness (VE) among adults aged ≥60 years

• Vaccination reduced the COVID-19 hospitalization risk by one-third overall

• Spring vaccine uptake was low, with shorter follow-up but VE like the fall analysis

• VE waned from about 40% at <8 weeks to negligible at ≥32 weeks post-vaccination

• COVID-19 VE against severe outcomes is meaningful but short-lived in older adults