I don't think anyone has really proposed this. Challenge trials are much more about getting quick answers to efficacy of treatment -- ethically, it's at least somewhat defensible to expose a volunteer if you also have medicine/vaccine for them, even if you're not sure how well it will work. It would be wildly unethical to give the virus to folks just to see how much virus it takes to get sick.

> I don't think anyone has really proposed this.

They seem to talk about it right near the top of major news stories, so I guess they are proposing it:

https://www.bbc.com/news/health-54612293

You might also get useful information about how people's immune systems react, when you give them too little of the virus to get infected. It's not only about figuring out the dose for the main trial. It also could have been done 6 months ago...

"It would be wildly unethical to give the virus to folks just to see how much virus it takes to get sick."

Depends. If there are volunteers for it, (which I can imagine to be the case) - and the volunteers are fully aware of the risk - then it might be ethical to let them proceed and save millions of other people.

Ethics are a bit more nuanced than "consent makes it OK". Suicide is consented to as well.

Thats why some people accept the right of other people over their own life.