IL2 and COVID-19: The RS between activation marker concentrations and the severity of COVID-19 are calculated by training data set Z2KP, where the activation markers are IL-17a, IL-2, GATA3, IFN-γ, IL-4, IL-10, PD_1, CD40L, RORγt, CTLA_4, CCR7, TNF-α and IL-6, respectively.