ACE2 and COVID-19: First, humoral and cellular responses showed a strong positive correlation with the severity of past COVID-19 (Figures 1, 3 and Supplementary Figure S1) (36) and the severe group had a higher proportion of ACE2-competitively RBD-binding memory B cells than the asymptomatic and mild groups (Figure 3), although the exact reason has not been clarified yet.