Background: The SARS-CoV-2 Omicron variant has replaced the previously dominant Delta variant because of high transmissibility. It is responsible for the current increase in the COVID-19 infectivity rate worldwide. However, studies on the impact of the Omicron variant on the severity of COVID-19 are still limited in developing countries. Here, we compared the outcomes of patients infected with SARS-CoV-2 Omicron and Delta variants and associated with prognostic factors, including age, sex, comorbidities, and smoking. Methods: We involved 352 patients, 139 with the Omicron variant and 213 with the Delta variant. The whole-genome sequences of SARS-CoV-2 were conducted using the Illumina MiSeq next-generation sequencer. Results: Ct value and mean age of COVID-19 patients were not significantly different between both groups (Delta: 20.35 +/- 4.07 vs. Omicron: 20.62 +/- 3.75; p=0.540; and Delta: 36.52 +/- 21.24 vs. Omicron: 39.10 +/- 21.24; p=0.266, respectively). Patients infected with Omicron and Delta variants showed similar hospitalization (p=0.433) and mortality rates (p=0.565). Multivariate analysis showed that older age (≥65 years) had higher risk for hospitalization (OR=3.67 [95% CI=1.22-10.94]; p=0.019) and fatalities (OR=3.93 [95% CI=1.35-11.42]; p=0.012). In addition, patients with cardiovascular disease had higher risk for hospitalization (OR=5.27 [95% CI=1.07-25.97]; p=0.041), whereas patients with diabetes revealed higher risk for fatalities (OR=9.39 [95% CI=3.30-26.72]; p=<0.001). Conclusions: Our study shows that patients infected with Omicron and Delta variants reveal similar clinical outcomes, including hospitalization and mortality. In addition, our findings further confirm that older age, cardiovascular disease, and diabetes are strong prognostic factors for the outcomes of COVID-19 patients.

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