"In a post-hoc analysis of the side effect questions of their original three-year RCT, they discovered 104 post-menopausal African American women given vitamin D were three times less likely to report cold and flu symptoms than 104 placebo controls. A low dose (800 IU/day) not only reduced reported incidence, it abolished the seasonality of reported colds and flu. A higher dose (2000 IU/day), given during the last year of their trial, virtually eradicated all reports of colds or flu." - Evan Parker

Evidence that vitamin D helps prevent the flu has been building for a while, but this paper is the first I've seen to show clear evidence of it in a controlled study and explain the mechanism behind it. The paper also goes into how flu is transmitted, suggesting that sick-to-well transmission is likely not the main way in which it is transmitted (I haven't read far enough yet to see if they suggest an alternative). - Evan Parker

Ah, towards the end it talks about "good transmitters" that are possibly asymptomatic being the primary method of transmission. - Evan Parker