Purpose: To explore the CT findings and pneumonia progression pattern of the Alpha, Delta, Omicron variants and subtypes of SARS-CoV-2 by comparing them with the pre-existing wild type.

Method: In this retrospective comparative study, a total of 15874 patients with COVID-19 were included: 1281 patients with wild type (748 men, 52.6 years), 7945 with Alpha variant (4972 men, 46.2), 4233 with Delta variant (2548 men) and 2415 with Omicron and subtypes (1301 man, 44.2). Chest CT evaluation included opacities and repairing changes as well as lesion distribution and laterality. Chest CT severity score was also calculated. These parameters were statistically compared across the variants.

Results: Ground glass opacity (GGO), crazy paving, subpleural fibrosis with consolidation and repairing changes were more frequent in the order of Delta variant, Alpha variant, Omicron variant, and wild type throughout the disease course. Delta variant showed GGO and crazy paving with consolidation more noticeable than did the others on days 1?8). Wild strain had more unclear CT presentation. Alfa strain was the first strain with sharp CT presentation. Delta strain was with fast development of subpleural fibrosis. Omicron strain had smaller areas of CGO. Total lung CT severity scores of Delta variant were higher than those of wild type on days 1?14 and that of Alpha variant on days 1?4. Alpha strain had higher severity score on days 15 to 21 with more complications. Omicron strain severity were mostly in range 0-5 and 18-23 with small percent in range 6-17.

Conclusions: There was difference in the CT findings and severity scores between all different types and subtypes.