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.
Source: Vitamin D and COVID-19: Lessons from Spaceflight Analogs | The Journal of Nutrition | Oxford Academic
Steve Parker, M.D.
PS: The Advanced Mediterranean diet promotes cold-water fatty fish consumption. But it’s not a requirement.