CD4 and COVID-19: In addition, differences between COVID-19 patients and close contacts were observed in the frequency of double-positive (IFNγ+ TNF+) CD4+ T cells (p < 0.0001 for CD4+, Supplementary Fig. 1a, p < 0.0001 for CD8+, Supplementary Fig. 1b), although CD4+, but not CD8+ cells producing both cytokines were significantly higher in close contacts than healthy controls (Supplementary Fig. 1a, b).