In agreement with the previous studies [48, 49], CSM exposure induced MUC5AC overexpression with further upregulation of MUC5AC expression after SARS-CoV-2 infection, suggesting that SARS-CoV-2 might aggravate mucus hypersecretion to cause further airway obstruction in smokers and COPD patients. This evidence concerns the gene MUC5AC and chronic obstructive pulmonary disease.