KRT18 and susceptibility to pneumonia measurement: In this study, we analyzed the histopathological features of the nasal passages, lung, and brains in experimentally SARS-CoV-2-infected K18-hACE2 Tg mice, hACE2 Tg mice, and hamsters, at 3 dpi, 5 dpi, and 7 dpi, respectively, when the most severe stages of pneumonia were seen.