PDCD1 and COVID-19: However, this observation has to be substantiated in future studies.71 Additionally, as assessed by flow cytometry of peripheral blood, T cells of severely affected patients are characterized by a much higher PD-1 expression than healthy controls.71 Specifically, enhanced PD-1 and Tim-3 expression on T cells was observed when patients progressed from prodromal to symptomatic stages, indicating that T lymphocyte exhaustion—similar to other viral infections—is a hallmark of COVID-19 as well.