I thought it might be helpful to recall how our immune system is designed to function in the age of Coronavirus.
The article linked above details the pathogenesis of measles and the way it hijacks our immune system and actually causes an immune memory loss to previous infection/exposure. It's all about T-cells, B-cells and other lymphocytes which first mount a defense against an invading virus and then remember it in order to defeat it quicker on next exposure.
We get vaccinated in order to develop ahead of time, a memory of the invading virus' proteins so that a ramp up of killer T-cells occurs within minutes of exposure. When a new virus enters the body, no such memory exists and the body has to first attach to the foreign virus and then create massive number of T-cells from scratch. This can take days to occur. Under normal new virus exposure, we become sick during this time as the body builds up the T-cell army to clear the new virus. On next exposure, this build-up phase is not needed as the memory T-cells are already in place and response is almost immediate.
What controls some people getting sick on first exposure and some people don't - or have very minor symptoms. It is the rate at which attacking T-cells can be generated and unleashed on the invading virus. Enter vitamin D.
We now know adequate levels of vitamin D is essential to activating T-cells to attack invading pathogens. Keep in mind, lasting immunity following an infection is of no use if the host dies while trying to mount a defense on first exposure. People who never seem to get sick, probably have an immune system with an ability to quickly recognize a foreign virus and then attack it very fast right at the beginning of first exposure. These people never show symptoms, although they could certainly be vectors of the disease in community spread until the virus is cleared from the body. I have a suspicion people who avoid sickness naturally - probably have higher than normal levels of vitamin D in their blood.
I can't stress enough that getting your vitamin D level checked regularly (twice a year) and then supplementing to make sure your vitamin D level is above 50 ng/ml (but less than 100 ng/ml) is a single most important way to inoculate yourself from coronavirus - or other virus'. No guarantee, we are all different in how we fight disease, but at least giving your body the tools to fight a virus on first exposure is a beginning. At least until there is a vaccine.
(p.s. I wonder how our elderly friends and family would be able to resist corona attack if instead of trapped inside buildings, were to be outside in the sun)