Lots of people here took vitamin D3 religiously and still got sick. I’ve been taking 5,000 IU /day for about 10 years.
Plant based or rock based? If it isn’t plant based, you are only gettin 8-12% absorption. Chelated might get you 22% max.
And getting sick isn’t the issue......hospitalization and dying is. My wife and I just shook off the A-strain of the flu without any problem.