We used DNA-barcoded peptide-major histocompatibility complex (pMHC) multimers to evaluate CD8+ T cell response to the BNT162b2 mRNA vaccine, including the impact of booster immunization, in patients with pre-existing hematological cancers (CLL, n=23; MDS, n=5; Supplementary Table S1) up to six months post-vaccination and compared with a cohort of healthy individuals (n=19; Supplementary Table S2) (Figure 1A). Here, CD8A is linked to myelodysplastic syndrome.