These analyses revealed the conspicuous presence of four cell populations that were uniquely present in the COVID-19 patients (albeit to different extents in the severe vs. remission disease stages), including a monocyte subpopulation (cluster 9), plasma B cells (cluster 11), effector CD8+ T cells (cluster 6), and proliferative MKI67+CD8+ T cells (cluster 12) (Supplementary Fig. 4c). Here, MKI67 is linked to COVID-19.