Fig. 3

Histopathological changes in the lung at 3 dpi in hamsters challenged with the Delta variant. a Representative images of the H&E-stained lung tissues of Group 1, 2, 3, and 4 hamsters at 3 days after Delta variant challenge. There was no obvious abnormality in the structure of the bronchus at any level. The alveolar wall was composed of a single layer of epithelium with a clear structure. No obvious inflammatory changes were observed (Group 1, prophylactic group). Lung tissue showed small foci of alveolar wall thickening with a small amount of neutrophil infiltration (black arrows); vascular congestion and dilation were common (green arrows) (Group 2, 3-dose antibody treatment group). Lung tissue showed mild alveolar wall thickening with neutrophil infiltration (black arrows); occasionally, there were a few necrotic epithelial cells, lymphocytes, and neutrophils in the bronchi (purple arrows) (Group 3, 2-dose antibody treatment group). Diffuse hemorrhage in the alveolar wall (red arrow) was seen in the lung tissue; a large number of blood cells in the bronchi were often seen (green arrow); multifocal thickening of the alveolar wall with neutrophil infiltration (black arrow) was seen (Group 4, control group without antibody). b Cumulative histopathological score of the lung lesions post Delta variant infection in hamsters. The mean along with standard deviation is plotted on the graph.