Cellular functionality was shown to be impaired in CD4 and CD8 T cells of critically ill patients, with reduced frequencies of polyfunctional T cells (producing more than one cytokine) as well as generally lower IFN-γ and TNF-α production following restimulation with phorbol myristate acetate (PMA) and ionomycin (Chen et al., 2020c, Zheng et al., 2020a, Zheng et al., 2020b).