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Purpose: COViK, a prospective hospital-based multicenter case-control study in Germany, aims to assess the effectiveness of COVID-19 vaccines against severe disease. Here, we report vaccine effectiveness (VE) against COVID-19-caused hospitalization and intensive care treatment during the Omicron wave. Methods: We analyzed data from 276 cases with COVID-19 and 494 control patients recruited in 13 hospitals from 1 December 2021 to 5 September 2022. We calculated crude and confounder-adjusted VE estimates. Results: 21% of cases (57/276) were not vaccinated, compared to 5% of controls (26/494; p < 0.001). Confounder-adjusted VE against COVID-19-caused hospitalization was 55.4% (95% CI: 12–78%), 81.5% (95% CI: 68–90%) and 95.6% (95%CI: 88–99%) after two, three and four vaccine doses, respectively. VE against hospitalization due to COVID-19 remained stable up to one year after three vaccine doses. Conclusion: Three vaccine doses remained highly effective in preventing severe disease and this protection was sustained; a fourth dose further increased protection. [ABSTRACT FROM AUTHOR]

• This study assesses detailed first-hand data and therefore provides a very high data quality. • Detailed interviews, abstraction of medical records and laboratory analyses were combined to obtain comprehensive data sets. • More than half of the cases (58 %) were not vaccinated at all, compared to 11 % of the controls. • The vaccine effictiveness to prevent severe disease in the Delta wave was 89 % overall for two doses and 98 % for three doses. We included 852 patients in a prospectively recruiting multicenter matched case-control study in Germany to assess vaccine effectiveness (VE) in preventing COVID-19-associated hospitalization during the Delta-variant dominance. The two-dose VE was 89 % (95 % CI 84–93 %) overall, 79 % in patients with more than two comorbidities and 77 % in adults aged 60–75 years. A third dose increased the VE to more than 93 % in all patient-subgroups. [ABSTRACT FROM AUTHOR]