As in the case of PBS (mock)-immunised and unchallenged K18-hACE2 mice (Figure 4A), H&E staining of lung tissue samples from challenged K18-hACE2 mice immunised with 2 × 103 pfu (Figure 4B) and 2 × 104 pfu (Figure 4C) of CoV-2-CNUHV03-CA22 °C showed mild pneumonia and no pneumonia, respectively. This evidence concerns the gene KRT18 and susceptibility to pneumonia measurement.