There seems to be mounting evidence that Vitamin D reduces the likelihood of contracting Covid and reducing the severity of it if you do get it. It has long been known to support the immune system and be effective against other respiratory infections.
You probably were getting more than sufficient Vitamin D from the sun during the summer but not so much now at the end of October. I've been taking supplements for the past month.
Years ago I used to see a podiatrist as I have arthritis in my big toe joints. It was always worse during the winter and he said this was down to the lack of Vitamin D from the sun. He was a big believer in supplements and suggested taking more than the standard dose (25 micrograms I think). He said it was difficult to overdose on Vitamin D and even if you did the side effects were minor.