Abstract

Introduction: This study is the first study in which demographic, laboratory data, and outcomes of coronavirus disease-2019 (COVID-19) patients due to the circulating SARS-CoV-2 infections caused by different variants (Alpha, Delta, and Omicron) are compared in Iran. Methods: We conducted a retrospective study of confirmed hospitalized COVID-19 cases from April 9, 2021, to May 22, 2022. Demographic data and laboratory findings were extracted from patients' electronic medical records on the first day of admission to the hospital. All patients were followed up for outcomes related to COVID-19 including intensive care unit (ICU) admission and mortality rate. Results: Of 760 confirmed hospitalized COVID-19 cases, 362, 298, and 100 represented patients during waves 4–6, respectively. During the Omicron wave, hospitalized patients were older than the other two waves and had a lower median level of C-reactive protein (CRP), alanine transaminase (ALT), aspartate transaminase (AST), and erythrocyte sedimentation rate (ESR). The median length of hospital stay during waves 4–6 was 5 days (interquartile range IQR: 4.0–8.0), 7 days (IQR: 6.0–11), and 6 days (IQR: 5.0–9.0), respectively (p < 0.001). The rate of ICU admission during waves 4–6 significantly increased. Conclusions: 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. © 2023 The Authors. Influenza and Other Respiratory Viruses published by John Wiley & Sons Ltd.