Particularly, some mucus components (e.g., MUC5B and MUC4) and genes related to mucus viscosity (e.g., TGM2 and TFF3) were significantly upregulated in COVID19 patients (Fig. 4J), as well as the mucus secretion score of club cells was significantly increased (Fig. 4K). Here, MUC5B is linked to COVID-19.