NK-cell–related cytokines and sCD25 in severe COVID-19. Serum concentrations of cytokines involved in NK-cell activation, IL12 (A), IL15 (B), and IL21 (C), in addition to sCD25 (sIL2-Rα; D), in severe COVID-19 patients were compared with those obtained among healthy controls using Mann-Whitney U statistics. Two-tailed P < .05 was considered statistically significant. (E) Association between sCD25 serum concentrations and percentage of NK cells in COVID-19 patients was evaluated by Spearman rank correlation. **P < .0001. nd, not determined (no detectable levels obtained in COVID-19 patients).