Vitamin D3
Get as much lunchtime sun in the next couple of weeks as you can, ideally in short shorts/vest top or bikini. 15 - 45 mins... stop well before you start to turn pink. After late September (UK) the sun doesn't reach high enough in the sky to let through UVB which produces vitamin D in your skin.
Eat wild salmon and oily fish several times a week.
Lose weight if overweight... the fat cells sequester and lock away vitamin D first, so there's very little actively available in the blood for immunological purposes. Try intermittent fasting and cutting out sugar, alcohol and processed carbs.
Take a supplement, a highish amount to start with eg 5000 iu, then after a few weeks drop down to around 3000 - 4000 iu... a bit more if you're in an at risk group... are darker skinned, are overweight/obese, live a mostly indoor lifestyle, or are in an older demographic, a bit less if you're fair skinned, slim, young and enjoy the outdoors. Ideally get your levels tested, you can get a publicly available postal test from NHS Sandwell for around £30. Optimum blood levels are around 100 - 150 nmol/l. Below 75, you're much more at risk from covid, and contracting a critical case of it. Talk to your doctor if you have a medical condition, especially something calcium related like kidney stones.
Download the DMinder app for phones, which can track your vitamin D intake from sun, food and supplements and estimate your current levels.
Pair D3 intake with K2, which you can get from animal foods like liver, gouda cheese, grassfed butter; a fermented Japanese soyabean food called natto, or supplements. K2 is crucial for regulating calcium metabolism in conjunction with D3, so it is correctly deposited in the bones and teeth and away from the arteries and soft tissue.
Zinc:
Take the standard recommended 15 mg per day. If you're concerned you've been exposed to cases, take 50 mg for a few days. Eat shellfish/liver/liver pate if you like it.
Ionophores:
Help transport the zinc into the cells. Eat lots of red onions, red apples, berries, colourful fruit and veg. Take a quercetin supplement. Buy some elderberry syrup, or make your own - there's loads in the hedgerows now. Take elderberry if you've been exposed to cases, or you feel you're starting to come down with something.
Vitamin C
Lots of fresh fruit and veg. Take a supplement 500 mg - 1 g per day. Make your own sauerkraut and kimchi (or buy ready made from the chiller cabinet).
Other stuff... ventilate rooms as much as possible to dilute any viral particles in the air.
Article about the first randomised control trial in relation to vitamin D and covid... it's very positive!
chrismasterjohnphd.com/covid-19/finally-confirmed-vitamin-d-nearly-abolishes-icu-risk-in-covid-19
Other info from watching scientific youtubers like John Campbell and Medcram.