I'd say while the header is to fight off colds & flu it is more heavily focussed on turbo-charging the immune system where it has already been compromised & weakened through other diseases as opposed to turbo-charging it as a matter of general practice.
As for vitamin D that's just cos we hide from the sun these days & even when out there we're covered in sunscreen blocking the uptake. Gone are the days people use to run around in their jocks like tribesman lol
