Introduction:Coronavirus disease 2019 (COVID-19) has been a pandemic since 2020 and depending on SARS-CoV-2 mutation, different pandemic waves have been observed.

Aims/objectives:To compare baseline characteristics of patients in 2 phases of the pandemic and evaluate possible predictors of mortality.

Methods:This is a prospective multicenter, non-interventional observational study, which included patients with C?VID-19 in 4 different centers in Greece. Patients divided into 2 groups depending on the period which they were infected during Delta and Omicron predominance.

Results:979 patients (433 Delta, 546 Omicron) were included in the study (median age 67 years [54,81];452[46.2%] female).Compared to Omicron period, the patients during the Delta period were younger (median age [IQR] 65 [51,77] vs 70 [55,83], p<0.001),required longer duration of hospitalization 8 [6,13] vs 7 [5,12] days, p=0.001, had higher procalcitonin levels (ng/mL) 0.08 [0.05,0.17] vs 0.06 [0.02,0.16], p=0.005 and ferritin levels (ng/mL) 301 [159,644] vs 239 [128,473], p=0.002.The Charlson Comorbidity Index was lower 3 [0,5] vs 4 [1,6],p<0.001 and and the extent of disease on computed tomography (CT) was greater during the Delta wave (p<0.001). No evidence of a difference in risk of death and admission to intensive care unit was found between 2 groups. Age, cardiovascular event, acute kidney and liver injury during hospitalization, extent of disease on CT, D-dimer value were identified as independent predictors of mortality.

Conclusions:In Omicron wave, patients were older with a higher number of comorbidities but patients with Delta variant had more severe disease and longer duration of hospitalization.