I have been supplementing with varying amounts of D3 since 2010. That year a housemate had come down with a bad flu and I handed him 4000IU as an "anti-inflammatory". The next day he knocked on my door ecstatically and asked what that was. His symptoms had nearly disappeared.

And in the past decade, I can count only a few times when I was struck hard by cold or flu, and they were when I had let supplementation trail off. When I'm having it daily, then the typical inflammatory symptoms only appear near or past the peak of the infection, and otherwise it just presents as tiredness and scatterbrained thinking, which I have learned to notice and pay some attention to.

I currently believe that some theraputic benefit may continue up to 8000 IU or so, but this is purely guesswork on my part, having tried amounts higher and lower. What I am confident in saying, having reviewed the research recently, is that there is some clinically studied, but poorly understood positive correlation between vitamin D supplementation and regulation of cytokine production, which is crucial for surviving the symptoms of respiratory diseases of this type.