CD4 and COVID-19: Antigen-specific CD4+ T cells expressing memory markers as well as IL-2, IFN-γ, TNF-α, and CD154 were markedly increased in COVID-19–recovered individuals compared with healthy donors, but the relative frequency of these cells decreased at the 6.1-mo time point (clusters 2, 3, 4, and 6; Fig. 3, A–D; and Fig. S3 A).