Multiple studies have shown that Vitamin D, as an effective immune system modulator, has lead to a reduction in the rate of acute respiratory infections. In one meta-analysis that included 25 randomized double-blind placebo-controlled trials with individual data from 10933 patients across the lifespan, vitamin D supplementation decreased risk of respiratory tract infections by 12%. Additionally, one study has shown that countries with highest rates of Vitamin D deficiency have the highest rates of infections and death while another study has shown that 75% of infected patients and 85% of those requiring ICU admission were Vitamin D deficient.
While discussing the studies of individual vitamins on COVID-19 is beyond this e-mail, it is important to know that nutrients are actively involved in the proper functioning and strengthening of the human immune system against viral infections including dietary protein, omega-3 fatty acids, Vitamin A, D, E, B1, B6, B12, C, iron, zinc, and selenium.
For more information, see article by Columbia University professor Dr. John P Bilezekian published in the European Journal of Endocrinology.