Abstract:Objective To investigate the clinical characteristics of patients with SARS-CoV-2 vaccine breakthrough infection and the risk factors for severe cases. Methods By the end of October 2021, the data of 55 patients with vaccine breakthrough infection after completely vaccinated with SARS-CoV-2 inactivated vaccine,who were caused by transmission chain in tourism in Gansu Province were retrospectively anlyzed, including 41 cases in the non-severe group（10 cases of mild type, 31 cases of common type） and 14 cases of severe type in the severe group. All the infected virus strains were identified as delta mutants. The clinical characteristics of all cases and risk factors of severe cases were analyzed. Results Compared with the non-severe group, the patients in the severe group were older, and the incidences of basic diseases such as diabetes, hypertension, coronary heart disease and chronic lung disease were significantly higher（P＜0.05）. In severe group,the incidence of dry cough and expectoration obviously increased, the number of blood lymphocytes decreased significantly, and levels of C-reactive protein（CRP） and creatine kinase isoenzyme-MB（CK-MB） increased more significantly compared with those in non-severe group（P＜0.05）. Multivariate logistic regression analysis showed that basic diseases were the independent risk factors for severe cases（β=-1.855, OR=0.156, P=0.018）. Conclusion Severe patients with SARS-CoV-2 vaccine breakthrough infection are older with more basic diseases, dry cough and expectoration, more obvious decrease in lymphocyte, and more obvious increase in CRP and CK-MB, which can be used as a warning signal for severe cases. The great attention should be paid to these patients,meanwhile, active control should be taken.