My immune system preservation team for the last 18 months

Modified Low-Carb diet (under 25 grams sugar a day, avoiding grains and seed oils)
5000 IU Vitamin D3, 
900mg - 1800mg Omega-3
200 mg magnesium bisglycinate once in am once at night
zinc, 
Melatonin at night 

I don't take these yet in capsule form but they're reputably helpful
K2 -- essential when one takes large doses of Vitamin D3, 
selenium, 
probiotics (capsule or fermented food), 
black seed oil

Dr. Robert Lustig ("The Hacking of the American Mind") [1]
Dr. Roger Seheult ("MedCram") [2]
Dr. Rhonda Patrick ("Found My Fitness" [3])

take variations of this supplement regimen. 
What's good enough for them is good enough for me.

1. https://robertlustig.com/metabolical/
2. https://blog.medcram.com/covid-19/vitamin-d-and-covid-19-the-evidence-for-prevention-and-treatment-of-coronavirus/
3. https://www.foundmyfitness.com/episodes/covid-19-episode-1
(and many others)