It has been identified in studies that selenium deficiency is a key factor in severe cases of COVID-19. Areas of land with soil low in selenium in China (a very specific belt of land in China) and Africa have shown to have the highest incidence of reported cases.
When selenium is deficient it’s easier for the virus to mutate. Viral mutation can make the virus more powerful and infectious. Selenium fights the virus by helping immune cells and the cells that eat viruses to do their job efficiently inside the body. It reduces viral symptoms by mopping up the oxidants produced by the immune system and infected cells, it slows the replication of the virus and prevents the mutation of the virus into new forms.
A 100mcg selenium supplement is recommend daily for immune support and 200mcg during infection. Take this alongside vitamin D (and vitamin K) and zinc to superboost your immunity against the virus. More on the effectiveness of vitamin D here and zinc here. Do always check with your doctor/NT before taking supplements if you are on medication. This is your responsibility.
Selenium is also available (levels could be questionable) in foods such as meat, fish, mushrooms (shitake, crimini), barley, eggs, brown rice, Brazil nuts, seeds: sunflower, mustard, sesame, flax, cabbage, spinach, garlic, broccoli and Swiss chard.