This article by a professor of surgery makes the case that Vitamin D in the range of 4,000-5,000 I.U. is safer and more effective than a flu shot.
Here's what Linus Pauling's site has to say about Vitamin D.
Here's the NIH fact sheet on it.
Here's an online vendor selling 5,000 I.U. in one capsule for less than 10 cents per.