PF4 and COVID-19: Cluster 3 (Fig. 3, B and C) revealed proteins whose levels increased with milder COVID-19, i.e., proteins linked to cholesterol metabolism [Padj = 1.8 × 10−2; example: apolipoprotein A1 (APOA1)], cell adhesion molecules [Padj = 1.3 × 10−3; example E-selectin (SELE)], viral protein interaction with cytokine and cytokine receptor [Padj = 1.5 × 10−3; example: platelet factor 4 (PF4)], and platelet degranulation (Padj = 1.7 × 10−13; example: platelet basic protein (PPBP)].