We estimated XBB.1.5 vaccine effectiveness (VE) against hospitalization/death among adults aged ≥50 years in Ontario, Canada, from September 2023 to June 2024. Compared with non-XBB.1.5 vaccinees, the initial protection at 0–3 months was 64 % (95 %CI, 57 %–69 %) during XBB-sublineage predominance. It was reduced to 57 % (95 %CI, 48 %–64 %) when JN/KP-sublineages became predominant, and quickly declined. No significant protection was observed >6 months post-vaccination. The VE estimates were lower when the last vaccine dose in the reference group was given more recently than 12 months, or was the omicron-containing bivalent vaccine. Short durability of protection poses unique challenges for COVID-19 vaccination.
Lee N, Nguyen L, Nasreen S, Austin PC, Brown KA, Buchan SA, Grewal R, Schwartz KL, Tadrous M, Wilson K, Wilson SE, Kwong JC; Canadian Immunization Research Network (CIRN) Investigators. Vaccine. 2025; May 26 [Epub ahead of print].
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