I've used Vit D3 plus k2 cause I was Vitamin D deficience. However, there are evidence that Vitamin D Supplementation could reduce risk of Influenza and COVID-19 Infections and deaths.
I take 5,000 iu Vitamin D3 from
vitaminexpress, one per day. Feel much more balanced. I actually take it before bed and I feel like I wake up more refreshed when I do.