Effectiveness of homologous/heterologous booster COVID-19 vaccination schedules against severe illness in general population and clinical subgroups in three European countries.

Using 4 data-sources (Spain, Italy, United Kingdom) data and a 1:1 matched cohort study, we aimed to estimate vaccine effectiveness (VE) in preventing SARS-CoV-2 infections with hospitalisations (±30 days) and death (±56 days) in general population and clinical subgroups with homologous/heterologous booster schedules (Comirnaty-BNT and Spikevax-MOD original COVID-19 vaccines) by comparison with unboosted individuals, during Delta and beginning of Omicron variants. Hazard Ratio (HR, by Cox models) and VE ([1-HR]*100) were calculated by inverse probability weights. Between December 2020-February 2022, in adults without prior SARS-CoV-2 infection, we matched 5.