Extended Data Fig. 7: Histological scoring of lung tissue of K18-hACE2 transgenic mice infected with SARS-CoV-2 WT, Delta, or Omicron.

Semiquantitative histology scores were given to each H&E image of the lung tissue by grading the severity of damage in bronchioles, alveoli, and blood vessels and accumulating the total scores. For bronchioles: 0 = normal structure; 1 = mild peribronchiolar infiltration; 2 = peribronchiolar infiltration plus epithelial cell death; and 3 = score 2 plus intrabronchiolar wall infiltration and epithelium desquamation. For alveoli: 0 = normal structure; 1 = alveolar wall thickening and congestion; 2 = focal alveolar space infiltration or exudation; and 3 = diffuse alveolar space infiltration or exudation or haemorrhage. For blood vessels: 0 = normal structure; 1 = mild perivascular oedema or infiltration; 2 = vessel wall infiltration; and 3 = severe endothelium infiltration. Data are mean ± SD (n = 3 for WT and Delta; n = 4 for Omicron). Statistical differences were determined with one-way ANOVA. Multiple comparisons among different groups were adjusted with Tukey’s multiple comparison tests. Images were obtained from two independent in vivo experiments. Each data point represents one biological repeat. * represented P < 0.05; ** represented P < 0.01; *** represented P < 0.001; **** represented P < 0.0001. ns, not statistically significant.