Differences between the course of SARS-CoV-2 infections in the periods of the Delta and Omicron variant dominance in Poland.

Introduction: Up to now, COVID‑19 caused more than 6 million deaths worldwide. So far, 5 variants of concerns have been identified, with Delta and Omicron being the subject of our analysis.
Objectives: We aimed to compare baseline characteristics and outcomes of patients hospitalized during the Delta and Omicron predominance in Poland.
Patients and Methods: The study population consisted of 2225 patients divided into 2 groups depending on the variant with which they were infected during the corresponding period of the pandemic.
Results: During the Delta wave, the median age of patients was significantly lower (65 vs 73 years; P <0.001), and the cohort was significantly less burdened with comorbidities than during the Omicron surge. The Omicron‑infected patients presented significantly less often in an unstable symptomatic state with SpO2 equal to or below 90% on admission (49.9% for Delta vs 29.9% for Omicron; P <0.001). Regardless of the pandemic period, the 2 most common early symptoms of COVID‑19 were fever and cough. In‑hospital treatment consisted of antiviral drugs, more frequently used in the Omicron wave, and immunomodulatory drugs, more frequently used during the Delta wave. The risk of mechanical ventilation was significantly lower in the patients infected with the Omicron variant (7.2% for Delta vs 3.1% for Omicron; P <0.001). For the age group above 80 years old, the risk of death was significantly higher during the Delta wave than during the Omicron wave. The risk of death was significantly lower in the patients treated with antiviral drugs regardless of the pandemic wave.
Conclusions: The Delta variant is associated with a more severe clinical course of the disease and a higher risk of death than the Omicron variant.