PRF1 and COVID-19: Importantly, the functionality of memory CD8+ T cells and CD8+PD‐1+CD38+ T cells in severe COVID‐19 patients was demonstrated by the high expression levels of perforin and granzyme B, compared with those from healthy controls,33 implicating that PD‐1 and CD38 act as activation T‐cell markers in COVID‐19 cases.