Between June 3 , and Dec 1 , 2011 , healthy children aged 2-14 years were randomly assigned ( 2:1 ) , by computer-generated permuted blocks of six with an interactive voice or web response system , to receive three injections of a recombinant , live , attenuated , tetravalent dengue vaccine ( CYD-TDV ) , or placebo , at months 0 , 6 , and 12 .