Cytostim-treated CD4+ T-cells in participants with mild COVID-19 1–3 months post-symptom onset, compared to those in participants recovered from other respiratory infections, had higher expression of the activation markers OX40, CCR4, CD69, and CCR6, and OX40 and CD69 expression remained elevated after 6–9 months post-symptom onset. Here, TNFRSF4 is linked to COVID-19.