He's now done a video on vit D relating to this type of infection, after watching it I've just taken a mega dose 4 x 75mcg, hopefully that'll help get my levels up [120 capsules 75mcg for £5@ amazon, better value than boots or H&barret I think]
If it helps to ward off this virus or reduce the severity if you do catch it, then it's got to be worth the small cost of 4.1p per day per capsule, health boosts don't come any cheaper than this. [I'm not affiliated in any way to amazon, other retailers are available

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I use to buy vitamins before but it can soon add up; Linus Pauling, the nobel prize winning scientist, [chemistry 1954] promoted the taking of Vit C and took mega doses of it himself. His research and theories on vit c were dismissed by the medical establishment, however he was a top scientist so I find it hard to believe he was making stuff up or not looking into the subject deeply with his intelligence.