These experiments suggest that 3 months after severe COVID-19, peripheral blood CD4+ and CD8+ T-cells are exposed in vivo to plasma factors that make them more responsive to IL-15 by inducing upregulation of the IL-15R, and that persistent T-cell activation observed ex vivo at 3 months may be driven by cytokines such as IL-15. The gene discussed is CD4; the disease is COVID-19.