Maybe. At vitamin D is relatively cheat and many folks are deficient in vitamin D yet don’t know it. From The Journal of Nutrition:
As others have mentioned, it is unlikely that one silver bullet will end the COVID-19 pandemic; however, evidence-based recommendations can be made that may reduce the risk of a severe response to SARS-CoV-2 infection or viral reactivation. Simpson and Katsanis have reported the benefits of exercising during the COVID-19 pandemic that was based on the evidence they found in their spaceflight research. We recommend that people maintain optimal vitamin D status to support immune function and lower their risk of viral reactivation, a recommendation that also comes from our National Aeronautics and Space Administration (NASA)–funded research. We are not advocating for ultra-high doses of vitamin D supplementation because of possible side effects, but rather a level of supplementation that will prevent vitamin D deficiency and maintain serum concentrations >30 ng/mL. We determined from our Antarctic research that doses of 1000–2000 IU/d, which are within IOM guidelines, are likely sufficient. Modifiable measures such as these may have the potential to safely and easily offer some protection and reduce risk.
Steve Parker, M.D.
PS: The Advanced Mediterranean diet promotes cold-water fatty fish consumption. But it’s not a requirement.