In this early acute phase, patients who progressed to severe COVID-19 were distinguishable from those who developed mild and moderate illness by their significantly higher mucosal CD40L, FGF-2, Flt-3L, IL-2, MDC, and VEGF, as well as systemic TNF-α, IL-10, IL-15, MIP-1β, IL-1RA, and IgA, and decreased systemic eotaxin (Fig. 6A and B). Here, FLT3LG is linked to COVID-19.