Ascorbic acid, or vitamin C, is an antioxidant, with a number of studies suggesting that vitamin C supplementation impacts the immune system. Additionally, have shown that vitamin C might protect against avian coronavirus infection, with human trials finding that vitamin C may decrease susceptibility to viral respiratory infections and pneumonia.
Vitamin D deficiency is common, with a lack of sun exposure, older age, corticosteroid use, and darker skin associated with lower concentrations of 25-hydroxyvitamin D. This deficiency is associated with a higher incidence of acute respiratory infections. It is also hypothesized that there is a link between seasonal influenza and vitamin D deficiency. Vitamin D supplementation has also been shown to decrease the incidence of acute respiratory infection. While it has yet to be studied for the prevention of COVID-19 infection, some recent articles have recommended taking daily supplements to raise 25-hydroxyvitamin D concentrations to reduce infection risk.
Zinc is known to be important for immune function. It has a role in antibody and white blood cell production and fights infections, while zinc deficiency increases inflammation and decreases the production of antibodies. High-dose zinc has also been found to reduce the duration of symptoms of the common cold. Because of its role in immune function and potential to decrease coronavirus replication, zinc is currently being used for the treatment of patients with COVID-19.