Peptide/HLA-E*0103 tetramer staining
Two peptides with high binding affinity S17 and S19 were selected for construction of peptide/HLA-E tetramer labeled with PE by Epigen Biotech (Nantong, China). The PBMCs of the SARS-CoV-2 vaccinated individuals were stained with each PE-labeled peptide/HLA-E tetramer for 10 min, and subsequently stained with FITC-labeled anti-human CD8 mAb (clone SK1, Biolegend, US) and APC-labeled anti-human CD3 mAb (clone OKT3, Biolegend, US). Approximately 300,000 cells gated on CD3+ cells were captured. The gate for CD8+ tetramer+ T cells was set up by matching non-tetramer staining isotype control (BioLegend). Compensation controls were checked regularly to avoid false-positive results and individually determined for each experimental setup.