PDCD1 and COVID-19: An inhibitory receptor related to the dysfunctional NK cell phenotype [75] has also been observed in chronic viral infections [74, 76] as well as in COVID-19 patients with other NK cell exhaustive phenotype molecules, such as lymphocyte-activation gene-3 (LAG-3), programmed cell death protein 1 (PD-1), mucin domain-3 (TIM-3), and T-cell immunoglobulin [77] (Fig. 1 and Additional file 1: Table S1).