In a phase 3 efficacy trial of a tetravalent dengue vaccine in five Latin American countries where dengue is endemic , we randomly assigned healthy children between the ages of 9 and 16 years in a 2:1 ratio to receive three injections of recombinant , live , attenuated , tetravalent dengue vaccine ( CYD-TDV ) or placebo at months 0 , 6 , and 12 under blinded conditions .