CD4 and COVID-19: It found that by 6 months after infection, peripheral blood CD4 + T cell, CD8 + T cell, and NK cell counts had returned to normal levels in most patients and remained stable from 6 to 12 months, and the activation of CD4 + T cell and CD8 + T cell persisted up to 12 months after infection in patients with mild and severe COVID-19 [24].