The possible underlying causes for the observed lymphopenia, especially the decrease in T cell counts, include T cell redistribution into infected organs, activation-induced exhaustion, apoptosis, and pyropoptosis [37]. While T cell exhaustion is observed in other viral infections [38], it seems to be more rapid, profound, and long-lasting in the setting of COVID-19. A recent study suggests that lymphopenia in severe COVID-19 patients is likely to result from SARS-CoV-2 infection of T cells in a spike-ACE2-independent manner [39]. Here, ACE2 is linked to viral infectious disease.