According to a study by Weiskopf et al. [16] in severe COVID-19 patients, the majority of CD4+CD69+CD137+ SARS-CoV-2-specific T cells were restricted to a central memory subtype and showed a high secretion of the effector cytokines IFN-γ, IL-2 and tumor necrosis factor (TNF)-α. Here, CD4 is linked to COVID-19.