If this is true, then it may be yet another reason for taking Vitamin D-2 in the winter or in overcast weather. Because sunlight on the skin is how the body normally makes Vitamin D.
There is certainly some evidence that Vitamin D helps cut down on colds and flu.
It might not be vit D though, but exposure to sunlight itself. Possibly due to UV, IR etc.
D3 is the better version of D to take. It is more similar to what the skin produces when exposed to sunlight.