ive had it once, an coincidently was the only flu season ive ever had the flu. We did work on this through uni, I think the results were that effective Vitamin D supplementation was masively more effective at preventing flu than the vaccine (think it was circa 30-40% more effective but dont holde too that- it was over 18 months since I looked into it lol)

I now take the 2000iu of D3 daily, helps with energy levels too