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Monovalent XBB.1.5 COVID-19 vaccine effectiveness against hospitalisations and deaths during the Omicron BA.2.86/JN.1 period among older adults in seven European countries: A VEBIS-EHR Network Study

Background Monovalent XBB.1.5 vaccine was administered among those aged >=65 years in EU/EEA countries in autumn 2023; soon after SARS-Cov-2 BA.2.86/JN.1 lineages became dominant. We aimed to estimate XBB.1.5 vaccine effectiveness (VE) against COVID-19-related hospitalisations and deaths during a period of BA.2.86/JN.1 predominance using a European multi-country study. Methods We linked electronic health record data to create historical cohorts in Belgium, Denmark, Italy, Navarre (Spain), Norway, Portugal and Sweden. We included individuals aged >=65 years eligible for the autumnal 2023 COVID-19 vaccine with at least a primary series. Follow-up started when >=80% of country-specific sequenced viruses were BA.2.86/JN.1 lineages (4/12/23 to 08/01/24) and ended 25/02/2024. At study site level, we estimated the overall vaccine confounder-adjusted (for age, sex, countries region, comorbidities and previous booster doses) hazard ratio (aHR) of COVID-19 hospitalisations and deaths between individuals with >=14 days after vaccination and individuals unvaccinated in autumn 2023, as well as by time since vaccination and stratified by age groups. VE was estimated as (1-pooled aHR)x100 with a random effects model. Results XBB.1.5 VE against COVID-19 hospitalisations was 50% (95%CI: 45 to 55) and 41% (95%CI: 35 to 46) in 65-79-year-olds and in >=80-year-olds respectively. VE against COVID19-related-death was 58% (95%CI: 42 to 69) and 48% (95%CI: 38 to 57), respectively, in both age groups. VE estimates against each respective outcome declined in all age group over time. Conclusion Monovalent XBB.1.5 vaccine had a moderate protective effect against severe COVID-19 likely caused by BA.2.86/JN.1 during the 2023/2024 winter, among persons aged >=65.