CD8A and infection: These donors had no infection history of SARS-CoV-2.Two-dose vaccinees had significant reduction in TNF-α + CD8 + T cells (Fig. 2b), but not IFN-γ + CD8 + T cells (Supplementary Fig. 9), when their PBMCs were stimulated with BA.1 Omicron, compared with Delta (Fig. 2b), indicating that only 2-dose vaccination could not prevent T cell immune escape caused by the BA.1 Omicron variant.