NK-cell quantity and function in severe COVID-19. Absolute NK-cell counts (A), percentage of CD56^dim cytolytic NK cells (of total NK cells) (B), and percentage of CD56^bright NK cells (of total NK cells) (C) of severe COVID-19 patients were compared with those obtained for healthy controls using Mann-Whitney U statistics. Upon stimulation with erythroleukemic K562 cells, the percentage of degranulating (CD107a⁺) NK cells (D), degranulation per NK cell expressed as CD107a mean fluorescence intensity (MFI; E), the percentage of IFN-γ (IFN-G)–producing NK cells (F) and IFN-G production per cell (IFN-G MFI; G) in severe COVID-19 patients was also compared. Two-tailed P < .05 was considered statistically significant. *P < .05; **P < .0001. ns, not significant.