Just read this, I can't say how accurate it might be.
Why don’t doctors recommend vitamin D for flu protection, especially when vitamin D activates the immune response far better than a vaccine? (http://www.naturalnews.com/027231_V…)
Answer: Because vitamin D can’t be patented and sold as “medicine.” You can make it yourself. If you want more vitamin D, you don’t even need a doctor, and doctors tend not to recommend things that put them out of business.
People still don't get it: Vitamin D is the "miracle nutrient" that activates your immune system to defend you against invading microorganisms -- including seasonal flu and swine flu. Two months ago, an important study was published by researchers at Oregon State University. This study reveals something startling: Vitamin D is so crucial to the functioning of your immune system that the ability of vitamin D to boost immune function and destroy invading microorganisms has been conserved in the genome for over 60 million years of evolution.
As this press release from Oregon State University(http://www.eurekalert.org/pub_relea...) explains:
The fact that this vitamin-D mediated immune response has been retained through millions of years of evolutionary selection, and is still found in species ranging from squirrel monkeys to baboons and humans, suggests that it must be critical to their survival, researchers say.
"The existence and importance of this part of our immune response makes it clear that humans and other primates need to maintain sufficient levels of vitamin D," said Adrian Gombart, an associate professor of biochemistry and a principal investigator with the Linus Pauling Institute at Oregon State University.