There have also been randomized controlled trials of Vitamin D vs placebo measuring flu rates. This is one meta-analysis I found: http://www.bmj.com/content/35

There have also been randomized controlled trials of Vitamin D vs placebo measuring flu rates. This is one meta-analysis I found: http://www.bmj.com/content/356/bmj.i6583 http://www.bmj.com/content/356/bmj.i6583