ObjectiveTo analyze the correlation between the vaccination status of inpatients with Omicron variant infection and the risk of Omicron critical illness. MethodsA retrospective analysis was performed on the clinical data of patients with Omicron infection admitted to a designated hospital for COVID-19 in Chengdu from December 1, 2022 to January 31, 2023. Patients were divided into critical group and non-critical group according to their condition and the "COVID-19 Diagnosis and Treatment Program (Tenth Edition)". According to the vaccination status, the patients were divided into incomplete vaccination group, full vaccination group and booster vaccination group. Multivariate logistic regression was used to analyze the association between vaccination, symptoms and signs at admission, and the risk of critical illness. ResultsA total of 3 603 inpatients with Omicron infection were included, including 730 cases (20.3%) in the critical group and 2 873 cases (79.7%) in the non-critical group. There were 2 399 people (66.6%) in the incomplete vaccination group, 433 people (12%) in the full vaccination group, and 771 people (21.4%) in the booster vaccination group. Compared with the incomplete vaccination group, the proportion of critical illness in the full vaccination group and booster vaccination group was lower, and the critical illness rate increased with age (P<0.05). After adjusting for age, gender, and underlying diseases, the results of multivariate logistic analysis showed that full vaccination (OR=0.67, 95%CI 0.50 to 0.89) and booster vaccination (OR=0.76, 95% CI 0.61 to 0.94) were significantly associated with a reduced risk of critical illness. ConclusionFull vaccination and booster dose can effectively reduce the risk of critical illness after infection.