They're finding out that one dose isn't enough... and kids are ending up with a more severe form of it as teens... that's why the recommendation for the booster. Here's an article on it:http://www.medpagetoday.com/Infectio...ccines/tb/5255
"Varicella vaccine efficacy drops dramatically over time, according to surveillance data that contributed to the CDC's decision last year to recommend a booster.
In the surveillance data, children vaccinated five or more years earlier were 2.6 times more likely to have a moderate-to-severe breakthrough case of chickenpox than those who had been vaccinated more recently.
The breakthrough varicella rate eight years after vaccination was 12 times higher than in the first year after vaccination even accounting for changes in exposure, reported Sandra S. Chaves, M.D., M.Sc., of the CDC, and colleagues. in the March 15 issue of the New England Journal of Medicine. .."