Abstract: Objective To investigate the clinical characteristics and laboratory tests of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta and Omicron variant infections cases, and to provide reference for the prevention and treatment of SARS-CoV-2. Methods A retrospective study was conducted to collect the clinical data of 23 cases of SARS-CoV-2 Delta variant admitted to Shenzhen Third People's Hospital from June 11, 2021 to November 8, 2021, and 34 patients with Omicron variant admitted from February 21, 2022 to March 9, 2022. We compared and analyzed the clinical symptoms, vaccination, laboratory test results, and chest CT imaging features of patients infected with different variants. Result s Among patients infected with the Delta and Omicron variants, 35 cases (61.4%) were males, and 56 cases (98.2%) were young and middle-aged. The main clinical manifestations were cough, sore throat and fever, clinical staging was predominantly mild (χ2=11.797, P＜0.05), and no severe cases. Comparing the two groups of patients receiving COVID-19 vaccine vaccination, it was found that the number of patients in Omicron group received booster shots of COVID-19 vaccine than Delta group (χ2=12.853, P＜0.05). The time from the last vaccination to the onset was more than 1 month in 90.9%（50 cases） patients, but there was no significant difference between the two groups (χ2=3.079, P＞0.05). In comparison of laboratory test results between the two groups, IL6 levels were higher while serum K and LDH levels were lower in the Delta group, with statistically significant differences (all P＜0.05). The median Ct value of viral nucleic acid in both groups at admission were lower than 20, and the nucleic acid Ct values in the Omicron group at day 3, 7, 14, and 21 were significantly higher than that in the Delta group (all P＜0.05), the number of viral nucleic acid negative cases at day 14 and 21 was significantly higher than those in the Delta group (all P＜0.05). The positive duration of viral nucleic acid and hospital stay were also significantly shorter than those in Delta group (all P＜0.05).In comparison of antibody levels between the two groups, the IgG positive rate of Omicron and Delta group at admission was 64.7% and 34.8%, respectively, the median level of IgM and IgG in Omicron group was significantly higher than that in Delta group (all P＜0.05). On day 7, the IgG positive rate of Omicron and Delta group was 93.7% and 75.0%, respectively, and there was no significant difference in the comparison of IgM and IgG between the two groups (all P＞0.05). Conclusion Patients infected with Delta and Omicron variants both have strong infectivity, and the Omicron variant infected patients with the virus's nucleic acid turned negative more quickly. Both may have certain immune escape against the existing COVID-19 vaccines.

Key words: Severe acute respiratory syndrome coronavirus 2, Variant, Delta, Omicron, Clinical characteristics