Previous observational studies report an independent association between low serum concentrations of 25 hydroxyvitamin D and susceptibility to acute respiratory tract infections Martineau et al. described vitamin D protection against acute respiratory tract infection overall. It was considered safe and protective.
Vitamin D deficiency impairs the ability of macrophages to mature, to produce macrophage-specific surface antigens, to produce the lysosomal enzyme acid phosphatase, and to secrete hydrogen peroxide, a functional integral to their antimicrobial functions. (Abu-Amer et al.)
Hypovitaminosis D is most pronounced in the aging population, which is also the population most susceptible to Covid 19. The cost of Vitamin D supplements is trivial compared to mainstream treatments with brand-new drugs released under EUA (emergency use authorization) that have been inadequately characterized.
References:
Martineau Adrian R, Jolliffe David A, Richard HL et al (2017) Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ 356:i6583
Abu-Amer Y, Bar-Shavit Z (1993) Impaired bone marrow-derived macrophage differentiation in vitamin D deficiency. Cell Immunol 151:356–368
MacLaughlin J, Holick MF (1985) Aging decreases the capacity of human skin to produce vitamin D3. J Clin Investig 76:1536–1538. https://doi.org/10.1172/JCI112134
Palacios C, Gonzalez L (2014) Is vitamin D deficiency a major global public health problem? J Steroid Biochem Mol Biol 144:138–145. https://doi.org/10.1016/j.jsbmb.2013.11.003