First, and most importantly, the variant of Dengue that they infected the (presumably somewhat healthy) person with was a weaker variant that is highly unlikely

First, and most importantly, the variant of Dengue that they infected the (presumably somewhat healthy) person with was a weaker variant that is highly unlikely to do serious harm, particularly in a hospital environment. Second, I know you probably know this, but the purpose of the placebo/control group is to understand if the vaccine is actually doing anything, or whether there is another common factor (possibly placebo effect, but possibly environmental, diet, poorly activated dengue virus, etc...) that is preventing Dengue from taking hold. I.E. if you have two groups one with the Vaccine, and one with just the Placebo, and both are injected with the dengue virus, and the group with the Vaccine and the group with the placebo both don't contract Dengue - then you have to consider the strong possibility that the vaccine didn't help, but some other factor prevented the two groups from getting dengue. And, it's far, far better that you try this with 10 people, with a relatively weaker version of Dengue, in a controlled hospital environment where the person can be taken care of, in an area where they are very likely to get the more dangerous dengue regardless of the study, than to come to the incorrect conclusion that you have a workable Vaccine, that you then spend many more hundreds of thousands (millions?) of dollars in broader clinical trials - only to discover the vaccine wasn't really doing anything in the first place.