IL9 and COVID-19: Immune cells, such as macrophages and T cells, release inflammatory mediators, mainly IL-6, IL-1, IL-10, IL-2, IL-8/CXCL8, IL-12, IL-5, IL-17, CD163, TRAIL, IL-15, TNF-β, IL-3, IL-4, IL-13, CXCL2/SDF-1, sTNFR1, leukocyte inhibitory factor (LIF), IFN-γ, macrophage inflammatory proteins (MIPs), IL-7, IL-9, RANTES/CCL5, CCL7, CCL8, CCL13, CCL17, CXCL10/IP-10 and CXCL11 which are associated with the poor prognosis of COVID-19.