Donnell, Carole A. Baggerly, Christine B. French, Jennifer L. Aliano and Harjit P. Bhattoa; Nutrients 2020, 12, 988; doi: 10.3390/nu12040988. 4 Vitamin D levels appear to play role in COVID-19 mortality rates, Amanda Morris; Northwestern University; May 7, 2020. 5 Martineau A.R., Jolliffe D.A., Hooper R.L., Greenberg L., Aloia J.F., Bergman P. et al. Vitamin D supple-mentation to prevent acute respiratory tract infec-tions: systematic review and meta-analysis of individual participant data. BMJ. 2017;
356: i6583. http://dx.doi.org/10.1136/bmj.i6583 .