TNFRSF9 and COVID-19: Although quite a lot of studies showed that T cells play important role in COVID-19 recovery, the phenotype and the functionality of SARS-CoV-2 specific T cells are not clear, so far, some studies focus on the phenotype defined by CD137 and CD69 (AIM) for virus specific CD8+ T cells, and CD154 and CD137 for CD4+ (11, 27), some studies sticked to the phenotype defined based on IFN-γ+ and TNF-α (ICS) (15).