We found that cells from patients who had recovered from severe COVID-19 have significantly higher cytotoxicity gene expression scores (Fig. 2a; P = 0.00032), with upregulation of GZMK (P = 3.02 × 10−5) and GNLY (P = 1.41 × 10−9) (encoding granzyme K and granulysin, respectively) (Fig. 2b). Here, GZMK is linked to COVID-19.