Challenge trials also don't provide as much useful information.

Nobody is getting covid on purpose, in a lab. Being protective or unprotective against a lab dose isn't the same as being protective or unprotective against a real-world dose.

We don't really know what the "real-world" dose is.

I think the first goal of these challenge studies is to figure this out. Give slowly increasing doses and see what the threshold is.

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.

And of course:

1. You have to have a control group, so half of test subjects would be exposed to the virus without having received the vaccine.

2. The highest-priority recipients for a vaccine are members of high-risk groups; it'd be pointless to run a challenge trial full of low-risk individuals.

So a challenge trial wouldn't just mean giving the virus to a few thousand vaccinated macho 20-year-olds. It'd mean giving the virus to unvaccinated 80-year-old care home residents.

> it'd be pointless to run a challenge trial full of low-risk individuals.

This is binary all-or-nothing thinking. It considers 90% the same as 0%, since both are not 100%.

Testing on healthy young people you learn how a normal immune system reacts to a vaccine candidate. High risk groups mostly have similar immune systems.

Even if somehow you could only vaccinate everyone under 60, that would do enormous good stopping the spread.

Another way of making the point: we did COVID challenge trials on monkeys first, because their immune systems are a decent model for that 80-year-old human's. Well, a 30-year-old human is an even better model of an 80-year-old human. A challenge trial on young people wouldn't prove everything, but it would give a high-information signal quickly.

> This is binary all-or-nothing thinking. It considers 90% the same as 0%, since both are not 100%.

Thinking is not just "binary or not"; there are degrees. If exposing that group of people would be 90% pointless, it's not much less out of the question than if it were 100% pointless.

I like your recursive "degrees of binarity" argument :)

You're right about the principle, of course. I just think it would be "10% pointless", and thus well worth doing.