So, we know people get Dengue fever and what it does to people. Why do you still need the placebo group in a study like this? I'm having a hard time seeing the morality of such a choice when we already have plenty of data on Dengue fever.
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.
The point of using an attenuated virus is that this is a "challenge model" that is supposed to simulate an exposure to the real virus closely enough that one can judge if the vaccine is likely working, without exposing the patients to the risks of infection by the full-strength virus. CNN did a much better job than Wired in explaining this. http://www.cnn.com/2016/03/16/health/dengue-vaccine-effectiv...
Therefore, the fact that the placebo group had certain levels of various symptoms (according to the article, 80% had a rash, 20% had nausea, etc.) is still important data for other scientists who are trying to judge if they should use the rDEN2Δ30 challenge model in their candidate vaccine trials. It's not intended to provide data on what natural dengue fever is like, which I agree would be unethical and unnecessary.
In a cancer trial you'd typically compare standard of care with the new drug.
Plus, there are some cancers out there with very few working treatments (so no real SoC) - If SoC is not available you would not consider a placebo group anyway.
What I haven't seen is a clinical trial without a control arm.
In terms of the ethical considers it's not unusual to run a trial for a yeat and then switch every patient over to the new drug.