Importantly, there were statistically significant increments in NK cells expressing the CD69 C-type lectin (Fig. 2a), a marker of NK cell activation, and of NK cells expressing the checkpoint molecules TIM-3 (Fig. 2b) and PD-1 (Fig. 2c), whereas the frequencies of NKG2D-, Siglec-7-, DNAM-1-, and CXCR6-expressing NK cells were significantly reduced in COVID-19 patients (Fig. 2d–g). Here, PDCD1 is linked to COVID-19.