ART4 and infection: Our results are also consistent with a similar study that also used ELISpot IFN-γ assays and S glycoprotein peptide subpools to measure responses of fresh PBMCs from HCWs who received a single or two doses of the Pfizer-BioNTech BNT162b vaccine and were either previously infected or uninfected with SARS-CoV-2 [43]; after vaccination, summed S-specific ELISpot IFN-γ responses were significantly higher in previously infected HCWs than in infection-naïve participants, and responses were distributed among the four peptide subpools that spanned the S1 and S2 subunits.