TNFRSF9 and infection: Activation induced marker (AIM) assays (namely CD137+ and OX40+ for CD4+ T cells and CD69+ and CD137+ for CD8+ T cells) have been used in a number of SARS-CoV-2 studies to measure T-cell memory to SARS-CoV-2 for up to 8 months after infection [116] and for determining T-cell response to spike variants [117].