Comparison of clinical outcomes, demographic, and laboratory characteristics of hospitalized COVID-19 patients during major three waves driven by Alpha, Delta, and Omicron variants in Tehran, Iran.

Although the Omicron variant caused less severe disease, in older patients who were hospitalized due to Omicron infection, longer hospital and ICU stays were reported, which could be attributed to their old age. In particular, elderly patients are more vulnerable to severe COVID-19; otherwise, as expected, other laboratory parameters and clinical outcomes were in accordance with differences in pathogenicity and infectivity of these variants.