Histologic lesions were semiquantitatively scored based on their severity (Fig. 2C); the significant difference identified at 2 dpi between MA10- and USA-WA1/2020-infected K18-hACE2 mice is attributable to the bronchiolitis that developed exclusively in the former group at 2 dpi (P = 0.029) (Fig. 2F). Here, KRT18 is linked to bronchiolitis.