Vitamin C does support the function of immune cells and enhances their ability to protect against infection. Vitamin C is a powerful antioxidant, protects against damage induced by oxidative stress, which can affect immunity. Supplementing with vitamin C has been shown to reduce the duration and severity of upper respiratory tract infections, including the common cold.
Studies have shown that vitamin D significantly decreased the risk of respiratory infections in people deficient in this vitamin and lowered infection risk in those with adequate vitamin D levels. This suggests an overall protective effect. Other studies note that vitamin D supplements may improve response to antiviral treatments in people with certain infections, including hepatitis C and HIV.
However, these are not guaranteed to prevent or cure COVID-19 infection specifically. Nutritional supplements should be not be considered to be substitutes for a good diet, because no supplements contain all the benefits provided by healthy foods.