TNF and respiratory tract infectious disorder: Though direct evidence of a link between vitamin D levels and COVID-19 incidence or outcomes is lacking, indirect evidence of an immunomodulatory role of vitamin D in respiratory infections exists, if only by the fact that it has suppressive effects on several inflammatory cytokines centrally implicated in fulminant COVID-19 illness, including interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and interferon-gamma (IFNγ).