compared the cytokines in serum and BALF of severe COVID-19 patients and found that both of them contained significantly elevated levels of CXCL1, CXCL8, CXCL12, CCL2, CCL3, CCL5, EGF, VEGF, and PDGF-AB/BB (45). Here, CXCL12 is linked to COVID-19.