This is totally unscientific BUT seems a reasonable thought to me.
Might Vitamin D be linked to Post Viral Syndrome? (I refuse to call it long Covid)
We know, that having a good immune system is protective and some research shows it reduces that chance of being seriously ill if you do get it.
I have done lots of reading about Vitamin D. And though none of it is conclusive. Vitamin D is cheap, readily available and there is little danger of overdose (Recommended dose is about 4000 x what NHS says is essential and overdose is another 40 times higher).
Overweight people are likely to have an even lower level than others as Vitamin D is fat soluble so less of what we do get is usable.
There is also research that shows that low Vitamin D has links with Depression, Fibromyalgia, MS, Dementia, Heart Disease, Schizophrenia, Heart Disease etc
"Symptoms of vitamin D deficiency include bone pain, muscle weakness, fatigue, and mood changes"
"found that school-age children who took vitamin D, compared with a placebo, were 42 percent less likely to get the flu."
cited article
I know I bang on about this but it is such a simple protection.