[RobAley]

(Source : I work in vaccine research, including challenge studies, in the UK, but only in IT/Digital, I'm not clinical).

In the UK at least (and as I understand it in most countries that subscribe to the principles of the Declaration of Helsinki), all of our studies have to have insurance to cover such eventualities (which are exceedingly rare these days). In addition, we provide clinical contacts for the participants throughout the trial and follow-up, and ensure that participant NHS medical records (and central NHS databases such as for the COVID vaccines) are updated with the necessary details for any future related care.

Money/Payment is always a careful balance. You don't want people taking part in the trials purely for the money (i.e. doing something they wouldn't otherwise be comfortable doing), but you need to ensure they aren't disadvantaged by taking part in the trial.