CD28 and infection: To assess the ex vivo phenotypes of CD4+ memory T cell subsets following COVID-19 vaccination and natural infection, we used the CD4+ T cell subsets gating strategy represented by Supplementary Figure S1 (CD45RA, CCR7, CD95, and CD28 are markers found on T cells: CD45RA marks naive T cells that have not encountered specific threats yet; CCR7 guides T cells to lymph nodes, where they can respond to threats; CD95 controls T cell survival and prevents overactive immune responses; and CD28 helps activate T cells by interacting with other immune cells).