https://www.pharmaceutical-technology.com/comment/vitamin-d-covid-19/
Vitamin D has a known effect on the immune system, as previous studies have found an association between susceptibility to respiratory tract infections and low vitamin D levels. Vitamin D levels being low have been associated with numerous factors including older age, less sun in the winter, darker skin pigmentation, exposure to less sunlight, and diet. Diets rich in vitamin D, such as those that include fatty fish, orange juice, cheese, and eggs yolks, can help prevent vitamin D deficiency, along with supplementing vitamin D.