CD4 and COVID-19: Therefore, we shortlisted 18 (12 studies13–24 for COVID-19 convalescent subjects and 8 studies1,14,20–22,25,26 for vaccinated subjects) research articles containing 89 different datasets (Supplementary Data 1) in which OX40 and CD137, CD69 and CD137 were used to measure spike-specific CD4+ and CD8 + T cells response respectively.