ROC curve analysis enabled us to identify that the KL-6 level of 453.5 U/ml is the cut-off value for the discrimination of COVID-19 patients and HS (area under the ROC curve (AUC) = 0.84 at 53% sensibility and 97% specificity, P < 0.0001) (Fig 1B). This evidence concerns the gene MUC1 and COVID-19.