CD4 and COVID-19: Antigen-specific CD4+ and CD8+ T cells have been found in rehabilitative patients.35 Braun’s group investigated a group of 18 COVID-19 patients and found reactive CD4+ cells (83%) in the blood from convalescent COVID-19 patients, which were specifically targeting the S protein.24 Meanwhile, another study found specific CD4+ T cells (100%) and CD8+ T cells (70%) in convalescent patients.31 Consistent with these studies, our study further confirmed the activation of CD4+ T, CD8+ T, and NK cells by stimulating the production of IFN-γ in vitro.