In a normal trial, you give half of participants a vaccine, and half of participants a placebo. Then, you wait around and see how many people in each group catch COVID naturally and get sick. Your vaccine works if fewer people who received a vaccine get sick compared to the placebo.

In a challenge trial, you give half of participants a vaccine and half of participants a placebo, and then purposefully expose them all to COVID so you don’t have to wait around for them to catch the virus naturally. As before, your vaccine works if fewer people who received a vaccine get sick.

A challenge trial gives you data which is more, not less, robust, because you’re controlling for more variables between groups. And we’ve used challenge trials to test vaccines in the past—just, never with a disease that’s nearly as deadly as COVID.

Any firestorm would result from a trial participant dying from the COVID they were purposefully given (which could absolutely happen), not from the vaccine. This has nothing to do with vaccine mandates.