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BACKGROUND The Delta variant has gradually replaced the Alpha variant as the major strain of SARS-COV-2 infection worldwide. METHODS We extracted the clinical characteristics and outcomes information about 381 hospitalized patients infected with Delta variant and compared them with 856 patients diagnosed with Alpha variant infection in Zhejiang Province. RESULTS The majority (85.3%) of patients infected with the Delta variant had received inactivated vaccine. The patients' condition was generally mild. Most of them were mild (35.7%) and common (62.7%) types. Only 6 patients (1.5%) were severe/critical types. During the follow-up period, patients infected with the Delta variant had longer hospital stays than the Alpha variant [24 (21-26) vs. 18 (14-24), P <0.001]. In addition, the unvaccinated patients infected with the Delta variant had a higher proportion of severe/critical cases than vaccinated patients (11.11% vs. 0.92%, P =0.024) and a higher usage rate of glucocorticoids (38.89 vs. 14.77%, P =0.017) and antibiotics (55.56% vs. 32.31%, P =0.042) during hospitalization. The vaccine's efficacy against severe COVID-19 did not diminish over time for patients who received two doses of the inactivated vaccine. CONCLUSIONS The disease types and clinical manifestations were generally mild in patients infected with the Delta variant, possibly associated with widespread vaccination with inactivated vaccines in China. This article is protected by copyright. All rights reserved.