> Is it? Yes > How exactly is vitamin D deficiency measured and diagnosed? Usually with a blood test. It's diagnosed because the blood test comes back out o

> Is it? Yes > How exactly is vitamin D deficiency measured and diagnosed? Usually with a blood test. It's diagnosed because the blood test comes back out of range which is usually below 30 ng/ml for insufficiency, and below 20 for a deficiency. > How much variance / tolerance is there between people? The std. deviation is 16.6 ng/ml > Most importantly does COVID impact Vitamin D levels? I don't think we know, it's hard to track this. You'd need to follow a huge cohort of people who you test for vitamin d often enough so that while they're getting the infection we see their vitamin d in real time.