摘要Objective: To determine the real-world effectiveness of the ChAdOx1 nCoV-19 coronavirus vaccine in preventing severe disease and mortality due to COVID-19. Methods: A case-case design was used to estimate the effect of the ChAdOx1 nCoV-19 coronavirus vaccine on severe SARS-CoV-2 outcomes in individuals aged 40 years and above. Cases (n=200) were COVID-19 patients admitted to intensive care unit (ICU) or who died. Controls (n=223) were those with mild COVID-19, fit for home isolation. The logistic regression model was used to estimate adjusted vaccine effectiveness for full vaccination (two doses ≥14 d) and partial vaccination status (one dose ≥14 d or two doses <14 d). Results: The proportion of fully vaccinated individuals was significantly lower among cases (12, 6.0％) compared to controls (30, 13.5％). The adjusted effectiveness of a full dose of ChAdOx1 nCoV-19 coronavirus vaccine in preventing ICU admission or death was 81.9％ (95％ CI: 61.3％-91.6％, P=0.001). Subgroup analysis restricted to age group, sex, and comorbidities found that ChAdOx1 nCoV-19 coronavirus vaccine had a significant positive effect in all subgroups and categories. Conclusion: COVID-19 vaccination reduces ICU admissions or death. Therefore, increased vaccine uptake may reduce the severity of the pandemic, more so in the elderly and those with comorbidities.