Higher levels of phosphorylated CD3ζ (Tyr83) (Fig. 4A) and ZAP70 (Tyr319) (Fig. 4B) were detected in pseudovirus–M protein–stimulated TREM-2+CD4+ T cells from patients with COVID-19 compared with TREM-2− groups, whereas pseudovirus vector did not induce the phosphorylations of CD3ζ and ZAP70 in TREM-2+CD4+T cells (Fig. 4, A and B). Here, CD4 is linked to COVID-19.