This paper is five years , so has nothing about Covid. However, nutritional yeast tweaks the immune system; thus, may help protect from Covid. It's taste reminds me of cheese.
Big Pharma have researchers scour the world for flora and fauna, from which they may extract and sell expensive drugs. Sometimes the common things in the human diet are good for us and sometimes not good for us. Vitamin D affects the immune system, and my PCP prescribed 7000 IU for me, but I can get more than that amount daily by sitting in the sun with my arms and legs exposed for half an hour. Why is it diffucult for anyone, including @AnneWimsey, to believe nutritional yeast contains chemicals that feed and improve our immune system?
I get vegan chick-pea flour "cheetos" , covered with Deeeelicious cheesy nutritional yeast, and fairly low salt, at Costco. But a preventitive/cure? Naah!