Thank you for your thoughtful comment. I agree with you that nutritional deficiencies are a global health concern that does not receive much attention or urgency. Vitamin D is one of the most important ones for Covid-19, as you have said, which I have written about before (link below if you are interested).

However, I think other factors are also important in influencing how one fares with Covid-19, such as genetics (e.g., toll-like receptor gene), viral dose (i.e., how much virus particles one inhaled), and stress levels (e.g., poverty stress). So, a hybrid or holistic approach to address these factors (including nutritional deficiencies) is indeed vital.

https://medium.com/microbial-instincts/is-vitamin-d-still-a-wonder-drug-for-covid-19-what-new-randomized-trial-and-existing-studies-say-a1237c7a18f5