Keskin , Olgun, Ayşegül Seremet Keskin, and Nilgün Seremet. “Association Between Low Serum Folic Acid and Vitamin B12 Levels With COVID-19 Prognosis”. Progress in Nutrition 24, no. 3 (September 27, 2022): e2022104. Accessed June 22, 2025. https://mattioli1885journals.com/index.php/progressinnutrition/article/view/13462.