I read a much better (and longer) article on this yesterday in the Washington Post (https://www.washingtonpost.com/news/to-your-health/wp/2016/03/16/scientists-

I read a much better (and longer) article on this yesterday in the Washington Post (https://www.washingtonpost.com/news/to-your-health/wp/2016/03/16/scientists-say-they-have-found-final-puzzle-piece-for-dengue-vaccine-a-breakthrough-that-could-help-in-fight-against-zika/ https://www.washingtonpost.com/news/to-your-health/wp/2016/0...). The interview with the researcher addressed pretty much all the points you are making and stated that the goal of the trial type was to demonstrate the efficacy against DENV-2 so that they would be able to move forward in a full trial of all four types. From the article: "Whitehead and the other researchers said that in the case of dengue, the testing was warranted because they knew that the vaccine appeared to be effective at preventing dengue 1, 3 and 4 viruses through previous testing but needed to learn more about its impact on dengue 2 before proceeding to larger trials that could take three to 10 years and cost tens of millions of dollars."