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 C, or L-ascorbic acid, is a water-soluble essential vitamin. It is a very popular dietary supplement due to its antioxidant properties, safety, and low price.
Vitamin C is often supplemented to reduce the symptoms of the common cold.
However, vitamin C is unable to reduce the frequency of colds in a healthy population. An athlete who frequently undergoes intense physical activity can expect to cut the risk of getting a cold in half. Supplemental vitamin C is able to reduce the duration of a cold by 8-14% in any population, when it is taken as a daily preventative measure, or at the beginning of a cold. Though superloading vitamin C (5-10g daily) is said to be more effective, further research is needed to determine the accuracy of this claim.
Vitamin C is capable of being both an antioxidant and pro-oxidant, depending on what the body needs. This mechanism allows it to serve a variety of functions in the body.
Vitamin C sequesters free radicals in the body. It is replenished by antioxidant enzymes, and is often used as a reference drug in antioxidant research. Vitamin C’s structure allows it to act on neurology and depression, as well as interact with the pancreas and modulate cortisol. Its antioxidant properties mean vitamin C provides neuroprotective effects and benefits for blood flow. By protecting the testes from oxidative stress, vitamin C can also preserve testosterone levels.