Earlier quoted context omitted.
Do separate branches then. Use challenge trials to accelerate deployment to under 30s (or whatever) and natural infection for other age brackets. I honestly haven't thought deeply about the ethics, but it's really concerning that it has become such a hard rule that preventing people from injuring themselves in a particular way (we don't prevent lots of other forms of injury) is somehow obviously superior to letting t…
The vaccine is going to be deployed to communities at risk first, you can't test vulnerable populations via volunteer challenge trials.
It won't hurt the at risk groups to start up high volume manufacturing earlier (it will likely help them), and there is a benefit to everyone each time anyone is vaccinated (the reduction in transmission efficiency).