Recent studies in patients with COVID-19 illness have reported that circulating CD8+ T cells express various activation markers such as CD137, CD69, PD-1, HLA-DR and CD38, most likely activated by SARS-CoV-2 infection in vivo (5, 9); these cells are also captured and contribute to an unbiased evaluation of virus-reactive T cells from patients with recent COVID-19 illness. Here, PDCD1 is linked to COVID-19.