The most important findings of the present study are: (1) IL-6 exerts prothrombotic effects on human endothelial cells because it promotes expression of the TF gene and TF protein with procoagulant activity, (2) IL-6 increases levels of soluble adhesion molecules, NLRP3 expression as well as ACE2r protein, suggesting that these might be potential mediators modulating the observed COVID-19 endothelial effects, (3) VitD significantly reduces these effects via modulation of NF-κB and STAT3 activation. Here, STAT3 is linked to COVID-19.