Increasing PD-1 expression on T cells was observed as disease progressed.97 The efficacy of PD-1 monoclonal antibody, camrelizumab plus thymosin have been evaluated in a clinical trial for COVID-19 treatment.443 Circulating NK cell numbers were found significantly reduced in COVID-19 patients with severe disease,444 and showed increased expression of inhibitory receptor TIM-3.445 Chimeric antigen receptor (CAR) -engineered NK cells are also being tested for treating COVID-19.446. Here, PDCD1 is linked to COVID-19.