Normally, to evaluate a vaccine like this, scientists would administer it to people at risk of infection and check back in after a few months to see how effectively the vaccine prevented it. For this study, the researchers used a different technique, called human challenge, in which they infect all patients—vaccinated or not—with a weakened form of the virus to see how well the treatment works. They injected 21 randomized patients with the vaccine, and their 20 control patients with a placebo. Six months later, the researchers dosed all the patients with a weak form of the dengue virus. They were looking to see if the patients had dengue-fighting antibodies in their systems, but also to see if they developed rashes or low white blood cell count, indicators of dengue infection. Among the patients not given the vaccine, 80 percent had a rash, and 20 percent had low white blood cell count. None of those given the vaccine showed signs of infection.