Yucel, H. E., Z. A. Erbesler, K. Tur, M. Celiker, N. Konar, and C. Ucar. “Vitamin D Deficiency and Clinical Severity of Covid-19 Infection”. Annals of Medical Research, vol. 29, no. 2, Feb. 2022, pp. 134-8, http://www.annalsmedres.org/index.php/aomr/article/view/4059.