It is invalid to extrapolate from efficacy against the common cold or respiratory tract infections broadly to the novel coronavirus in particular. For more information, see this page.
Vitamin D is a fat-soluble nutrient. It is one of the 24 micronutrients critical for human survival. The sun is the major natural source of the nutrient, but vitamin D is also found naturally in fish and eggs. It is also added to dairy products.
Supplemental vitamin D is associated with a wide range of benefits, including increased cognition, immune health, bone health and well-being. Supplementation can also reduce the risks of cancer, heart disease, diabetes and multiple sclerosis. People deficient in vitamin D may also experience increased testosterone levels after supplementation.
The body produces vitamin D from cholesterol, provided there is an adequate amount of UV light from sun exposure. There is only a sufficient amount of UV light coming from the sun when the UV index is 3 or higher, which only occurs year-round near the equator, between the 37th parallels.
Most people are not deficient in vitamin D, but they do not have an optimal level of vitamin D either. Due to the many health benefits of vitamin D, supplementation is encouraged if optimal levels are not present in the body.