Basically, Vitamin D regulates the immune system as I understand it. It basically keeps it in tune and more responsive to beginning infections. So basically it increases recognition of a pathogen and jumps on it faster if you have a good level of vitamin D in your system. If you don’t, then the immune system is slower to react and doesn’t react as well allowing the pathogen to multiply freely as the immune system slowly gets around to fighting the disease.
So by following this logic, as I understand it, is that I’d say Covid gets into your system, it starts to multiply. As there is no memory on how to fight this disease, the body has to start from scratch. Higher vitamin D will allow the immune system a quicker start to doing this and a quicker response once it figures it out. Someone lower in Vitamin D will allow Covid to continue to multiply and the infection to build up as the immune system trails behind.
Now I’m sure someone with more background can get much more into it, but that’s basically how it works.