ISG15 and viral infectious disease: This index was developed by selecting 8 genes (MX1, MX2, IFIT1, IFIT2, IFIT3, IFI6, ISG15, and ISG20) that were (a) previously known to be regulated at a transcriptional level by viral infection and/or IFN stimulation, (b) highly induced in our data set following SARS-CoV-2 challenge, and (c) consistently observed in all cell types captured in our analysis.