IL9 and influenza: Beyond the parallelisms between influenza and COVID-19 aforesaid, a compilation of retrospective data from independent studies indicate that IL-2 increases only during severe influenza, whereas high concentrations of IL-4, IL-7, IL-9, IL-17A, CCL4, CCL5, CCL7, CCL8, CCL11, GM-CSF, and PDGF are exclusive features of severe COVID-19 (Fig. 3A).