Clinical features and outcomes in adults with COVID-19 during the pre-Omicron and Omicron waves

•The death rate for Omicron inpatients (n = 1142) was 11 % vs 16.9 % for pre-Omicron inpatients (n = 1690)•Independent risk factors for death included unvaccinated status and comorbidities.•Type of variant was not an independent risk factor for death.•Omicron inpatients had fewer comorbidities but were more frequently immunosuppressed.•Nosocomial COVID-19 was more frequent during Omicron waves (18.2% vs 15.4%) We conducted a single-center retrospective study to compare patient characteristics and death rates during the Omicron (O, December 01, 2021, to September 30, 2022) and pre-Omicron (PO, March 01, 1920, to October 31, 2021) periods. We retrospectively analyzed the data of 2932 patients (1242 (O) and 1690 (PO)) hospitalized (>24 h) with laboratory-confirmed COVID. Compared to the PO period, O period patients were less frequently men, had a lower body mass index and fewer comorbidities except for immunosuppression and pregnancy. Nosocomial COVID-19 accounted for 18.2 % (O) and 15.4 % (PO) of cases (p = 0.05). Patient mortality rates during the O and PO periods were 11.0 % and 16.9 % (p