This study aimed to improve the clinical course of patients through rapid response by analyzing the characteristics of critically ill patients with confirmed coronavirus disease 2019 (COVID-19) in Busan between December 1, 2020, and December 31, 2021. We divided patients diagnosed with COVID-19 into mild-to-moderate and critical groups based on their clinical severity. The critically ill patients were further sub-divided into delta and delta variant non-epidemic group. The following factors were significantly more frequent in critically ill patients than in patients with mild-tomoderate symptoms: male sex, age ≥60 years, symptoms at the time of diagnosis, and those with underlying diseases. The following factors were significantly more common in the non-delta variant epidemic group than in the delta variant epidemic group in critically ill patients: male sex, age ≥60 years, underlying diseases, and not being vaccinated. In the delta variant epidemic group, the duration between confirmation of disease and its progression to critically ill status was significantly shorter than that in the non-delta variant epidemic group. COVID-19 is characterized by the emergence of new variants and repeated epidemics. Therefore, it is necessary to analyze the characteristics of critically ill patients to efficiently distribute and manage medical resources.