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It may well be a long way down the list of risk factors, which are all to stop doing things. Doing something proactively and proplylactically may make all the difference for some people. But then addressing individual differences are my expertise. I need to take it anyway, and there are a lot of people who are more at risk of deficiency than others eg elderly people and people with darker skin
The stats say 20% of adults have some degree of Vitamin D deficiency, dark winters and all. When it comes to evidence-led advice, I'd prefer to rely on statistics than gut reaction and instinct. Of those, a percentage will have their immune systems improved by a significant extent by taking Vitamin D supplements. Of that smaller percentage, a still smaller percentage will find their recovery from Covid19 improved by taking supplements. Sure, it won't hurt taking them, but in the grand scheme of things it's a long way down the list of risk factors. Which is exactly the original point.