Vit. D because of a deficiency.
I don't see the use of blindly taking all kinds of vitamins without having a reason to suspect you don't get enough of them through food.
Vitamin d3 & zinc from start of September to April
D3 for the sunshine
Zinc to stop coughs & colds (as recommended by a friend who’s a chest consultant)
Vitamin D is an important one. According to some, it played a huge role in Covid recovery (or not).
There are numerous articles if you Google Vitamin D and immunity.
Here's one: https://www.hsph.harvard.edu/nutritionsource/vitamin-d/
I read that 90% of our Vit D comes from direct sunlight on our skin, 10% through diet.
(I take 6000 iu supplement per day in the non-sunshine months in the UK)