This supplementary material includes the NIHR BioResource collaborators, figures S1-S9, tables S1-S5, and references. The figures show that patients with Long COVID produce more interferon gamma (IFN-γ) and pro-inflammatory cytokines spontaneously, even without stimulation. These findings suggest persistent T cell activation in Long COVID patients. Figure S1 shows increased IFN-γ production in Long COVID patients compared to unexposed controls. Figure S2 shows no difference in spontaneous IL-2 release between Long COVID patients and controls. Figures S3-S6 show that Long COVID patients do not show general T cell dysfunction when stimulated with viral peptides or positive controls. Figure S4 shows that unstimulated IFN-γ release does not correlate with the severity of acute COVID-19. Figure S5 shows that Long COVID patients have altered immune cell composition. Figure S6 shows that Long COVID patients produce more pro-inflammatory cytokines spontaneously. Figure S7 describes the gating strategy used to identify CD4+ and CD8+ T cells. Figure S8 shows increased IFN-γ production in Long COVID patients compared to healthy controls. Figure S9 shows that Long COVID patients produce more IL-2 in response to spike peptide stimulation after vaccination. Tables S1-S5 provide additional data on Long COVID symptoms, antibody panels, and IFN-γ secretion. References include studies on Long COVID, immune responses to SARS-CoV-2, and other related topics.This supplementary material includes the NIHR BioResource collaborators, figures S1-S9, tables S1-S5, and references. The figures show that patients with Long COVID produce more interferon gamma (IFN-γ) and pro-inflammatory cytokines spontaneously, even without stimulation. These findings suggest persistent T cell activation in Long COVID patients. Figure S1 shows increased IFN-γ production in Long COVID patients compared to unexposed controls. Figure S2 shows no difference in spontaneous IL-2 release between Long COVID patients and controls. Figures S3-S6 show that Long COVID patients do not show general T cell dysfunction when stimulated with viral peptides or positive controls. Figure S4 shows that unstimulated IFN-γ release does not correlate with the severity of acute COVID-19. Figure S5 shows that Long COVID patients have altered immune cell composition. Figure S6 shows that Long COVID patients produce more pro-inflammatory cytokines spontaneously. Figure S7 describes the gating strategy used to identify CD4+ and CD8+ T cells. Figure S8 shows increased IFN-γ production in Long COVID patients compared to healthy controls. Figure S9 shows that Long COVID patients produce more IL-2 in response to spike peptide stimulation after vaccination. Tables S1-S5 provide additional data on Long COVID symptoms, antibody panels, and IFN-γ secretion. References include studies on Long COVID, immune responses to SARS-CoV-2, and other related topics.