Our results show that patients with severe COVID-19 exhibit significantly higher percentage of NK cells positive to the functional markers IFN-γ (31.9% [2.9-89.3]) compared to the control group (2.15% [0-45.6]), P = 0.010; and Granzyme B (51.8% [16.8-77.2]) compared to the control group (28.2% [24-44]), P = 0.015. Here, GZMB is linked to COVID-19.