Therefore, we should first pay attention to whether the KL-6 level of patients with secondary pulmonary fibrosis exceeds 674 U/mL, which reflects irreversible fibrosis during the process of SARS-CoV-2 infection, and then judge whether the KL-6 level decreases after PCR turns negative, which reflects gradual absorption of pulmonary fibrosis. Here, MUC1 is linked to pulmonary fibrosis.