Higher expression levels of NKG2A on NK cells and CD8+ T cells were observed in severe cases of COVID-19 than that in mild cases, which means part of their functions were compromised in the face of viral infection; meanwhile, decreased secretion levels of IFN-γ, IL-2 and TNF-α were also noticed on NK cells and CD8+ T cells (20). Here, IL2 is linked to COVID-19.