design: PBMCs at 1 day pre-challenge and 28 days post challenge from 23 naive subjects (13 HVL, 10 LVL), and 14 vaccinated subjects (7 with the highest and 7 with the lowest magnitude of T cell responses to NP and M1 peptides at Day 28 following vaccination) were selected for single-cell RNA-Seq and single-cell ATAC-Seq assays