Studies have shown that vitamin D dosing 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.