Fig. 3: Histopathological findings in wild-type hamsters infected with Omicron subvariants.

Male hamsters (n = 5 per virus/timepoint) were infected with 10⁵ PFU (30 μL total volume) of BA.4 (UW12575 and UW12767), BQ.1.1, XBB, BA.5, or B.1.617.2. Tissues were collected at 3 dpi (a) or 6 dpi (b) for histopathological examinations. Representative images of the bronchi/bronchioles and alveoli at high magnification are shown. Upper rows, hematoxylin and eosin staining. Middle rows, immunohistochemistry with a rabbit polyclonal antibody that detects SARS-CoV-2 nucleocapsid protein. Lower rows, in situ hybridization targeting the nucleocapsid gene of SARS-CoV-2. Scale bars, 100 µm. c Histopathological scores of inflammation in alveolar regions for all animals examined are shown. The scores were determined on the basis of the percentage of alveolar inflammation in a given area of a pulmonary section collected from each animal in each group using the following scoring system: 0, no inflammation; 1, affected area (≤1%); 2, affected area (>1%, ≤10%); 3, affected area (>10%, ≤50%); 4, affected area (>50%); an additional point was added when pulmonary edema and/or alveolar hemorrhage was observed. Therefore, histopathological scores of inflammations in alveolar regions for individual animal range from 0 to 5.