TBXT and COVID-19: Due to their high avidity for their cognate T-cell receptors, they have been systematically used to visualize and quantify low frequency antigen-specific T cells ex vivo using flow cytometry.44,45 We focused on four SARS-CoV-2 epitopes (CVADYSVLY, HVGEIPVAY, NQKLIANQF, and RVAGDSGFAAY) previously shown to elicit cellular immunity mediated by cytotoxic CD8+ T cells in COVID-19 patients carrying HLA-B*15:01.46–50 Next, we performed ex vivo pMHC tetramer evaluation with the SARS-CoV-2 peptides to detect antigen-specific CD8+ T cells in nine pre-pandemic PBMC samples.