PDCD1 and COVID-19: Lymphocytopenia, as well as reduced polyfunctionality and cytotoxicity of T-cells and NK cells due to the continuous expression of inhibitory markers such as programmed cell death protein-1 (PD-1) and T-cell immunoglobulin domain and mucin domain protein-3 (TIM-3), are characteristics of both influenza infection and COVID-19 [19, 20].