Actually, this sort of vaccine “challenge test” is usually only done on animals.
Statistics are stated various ways. Like 99.xx% survival rate for young.
OTOH, of all those who have died of the disease — 80% were over age 65. 20% were younger. 20% seem rare to you?
At first yes. But because none of the animals have the exact same reactions as humans, either to vaccines or illnesses, eventually you have to test on humans. There’s no way to 100% know how it will do in humans until you do. Even with all the really cool modeling and cellular tech we have. In the end biology is just has too many variables.