We estimated the effectiveness of the adapted monovalent XBB.1.5 COVID-19 vaccines against PCR-confirmed SARS-CoV-2 hospitalisation during the BA.2.86/JN.1 lineage-predominant period using a multicentre test-negative case-control study in Europe. We included older adults (≥ 65 years) hospitalised with severe acute respiratory infection from November 2023 to May 2024. Vaccine effectiveness was 46% at 14-59 days ...