Moreover, there was also no significant difference in other T4 lymphocytes, such as naïve T4 cells, effect memory T4 cells, effector T4 cells, CD28− differentiated T4 cells, CD57+ differentiated T4 cells, and PD1+ depleted T4 cells, between mild, severe and critical COVID-19 patients (Fig. 3B). Here, CD28 is linked to COVID-19.