I am taking Vitamin D , 2000ui per day, for a few years now, and it has been very beneficial for me. When I had my 25 OH-D tested, it was fine, at 109.
Last year, I switched to the new Jamieson 2500ui tablet, one daily. I live quite up north - long winters and not much sun. Most Canadian do not have optimal Vit D levels ( 56% do not hit the 75 minimum target, so most have at least a slight deficiency).
Vitamin D is very good for the immune system. A few weeks ago, I have been in isolation for a week with my spouse and my daughter who both had COVID. I didn't catch it, even if we did not wear masks in the house, and I did not hide in the basement either. I had 2 shots of covid vaccine, just like they had, but they caught it - I guess my daughter had it first, and my spouse got it from her. But not me. I tested negative on a rapid test when they first tested positive, and I still tested negative after 5 days of isolation with them ( here, we had to stay isolated when a member of our family has Covid, even if personally testing negative and having no symptoms). When I was younger, I was catching everything... so, this is surely an improvement, and I think that vitamin D is responsible at least in part for that better immune protection I have.
I took some Moducare, also, during this week. Moducare is great for my immune system too. I also took more vitamin C than usual, and also zinc. And of course, my usual 2500ui of vitamin D daily.