(A) Cellular immune response to wild-type spike by ELISpot assay (Pre, n = 23; Post, n = 45): IFN-γ SFCs after subtraction of MOG. Statistics performed using Mann–Whitney U test: p = 0.002. Negative responses: Pre, 8; Post, 5. (B) Cellular immune response to wild-type and Delta spike in AIM assay (n = 14): percentage of CD4+ CD137+ OX40+ T-cells after subtraction of DMSO. Dotted line shows the LLoD at 0.001%. Statistics between pre- and post-vaccination for WT in PLWH performed using Wilcoxon matched-pairs signed rank test (p = 0.005); pre- and post-vaccination for Delta: ns. Statistics between PLWH and controls with Mann–Whitney U test: not significant. (C) Cellular immune response to wild-type and Delta spike in AIM assay (n = 14): percentage of CD8+ CD137+ CD69+ T-cells after subtraction of DMSO. Dotted line shows the LLoD at 0.001%. Statistics performed using Wilcoxon matched-pairs signed rank test (p = 0.008); pre- and post-vaccination for Delta: ns. Statistics between PLWH and controls with Mann–Whitney U test: not significant. Green circles: mRNA vaccines; blue squares: vector-based vaccines. Pre: before vaccination; Post: 4–6 weeks after second vaccination. AIM, activation-induced marker; BAU, binding antibody units; cART, combination antiretroviral therapy; DMSO, dimethyl sulfoxide; ELISpot, enzyme-linked immune absorbent spot; GMC, geometric mean concentration; INF-γ, interferon-γ; LLoD, lower limit of detection; MOG, myelin oligodendrocyte glycoprotein; PBMC, peripheral blood mononuclear cell; PLWH, people living with HIV; SFC, spot-forming cell; WT, wild type.