Severity of Omicron BA.5 variant and protective effect of vaccination: national cohort and matched analyses in Scotland

The BA.5 sub-lineage was first detected in South Africa in January 2022. It was designated a variant of concern by the UK Health and Security Agency on May 18, 2022, and became dominant in the UK by June 24, 2022. There are few longitudinal population-based studies on the risks of infection and serious COVID-19 outcomes of BA. A case control and cross-sectional study in Denmark found that BA.5 was associated with an increased risk of COVID-19 hospitalisation compared to BA.2. A test negative case–control study in England found no evidence of reduced vaccine effectiveness (VE) against hospitalisation for BA.5 compared to BA.2. In this study, we analysed severity of the BA.5 variant compared to BA.2, which was the most recent dominant Omicron sub-lineage in the UK prior to BA.5.