Most important among them is to optimize one's vitamin D levels (50-80 ng/L) and to support that with magnesium supplementation. Vitamin D has been shown to protect us against most forms of solid organ cancers, heart disease, heart attack, and strokes. Covid-19 patients that have lower levels of vitamin D do not do well and suffer more serious symptoms of disease, even death. Omega-3 fatty acids in the form of fish oils or krill oil do similar things. So the "Big Three" that I rely on regularly are vitamin D, magnesium, and omega-3 fatty acids.
Specific for viral infections, including covid-19, I supplement with zinc and add quercitin. Zinc inhibits a virus' ability to replicate but is hard to get into cells. Quercitin shuttles zinc into the cell where it can do its magic.
Add this strategy to common sense non-medical procedures such as washing your hands, hydrating, getting rest, cutting down on sugar and alcohol (not to mention smoking of any kind!), exercising regularly. If you do this your immune system will take good care of you. You might get sick, but your symptoms will be milder. Chances are you will be fine. Years ago I have struggled with sinus infections, colds, and bronchitis every winter UNTIL I adopted the above approach.
My office has been open during the entire pandemic, treating other health care providers and emergency patients. During that time I have been exposed to covid-19 persons, have been on an airplane to Florida since last year several times helping with an ailing parent, and through it all I have not contracted covid-19, a cough, a cold, nor a sinus infection. I attribute this to the above strategy and getting regular chiropractic care, which effectively stimulates a healthy immune system. The bottom line is that I have faith in my body's ability to fight off disease. Experience has shown me that as long as I take care of my body, it will take care of me.