Compared to healthy controls or patients with other pulmonary infections patients with severe COVID-19 display elevated markers of platelet activation including increased thromboxane A2 (TXA2) release (12, 15), surface expressed CD62P and CD63 (12, 15–18), activated glycoprotein (GP) IIb/IIIa (12, 19, 20), decreased intraplatelet serotonin (5-HT) and platelet factor 4 (PF4/CXCL4) (21) with increased plasma levels of 5-HT, PF4, and soluble CD40 ligand (sCD40L) (19, 21). Here, PF4 is linked to COVID-19.