To add some context to this, medical trials generally only compensate time (at a low hourly rate) and expenses, occasionally increased slightly to compensate for hardship.

They deliberately do not pay large amounts, because the money is not supposed to overly-incentivize the participation, to avoid ethical issues where someone might choose to participate in something very unsafe only because the reward seems so large.

I did various medial trials in college for money and have mixed feelings about this. When sitting in the various waiting rooms, it was very clear that was only one kind of person that did "open" (not targeting people with a disorder but rather recruiting healthy volunteers) medical trials: the poor. I got to know lots of of people well (we were frequent flyers in the medical trial world), and our common characteristic was that we really needed the money.

It's kind of like non-pay electoral offices. You only get candidates that can afford not to work. In medical trials, you get the kind of people that are willing to, for example, get infected with the common cold and have you blood drawn 6x a day for 4 days for about 80 bucks a day. It was not horrible, but often uncomfortable. Study recruiters still call me, and now that I have a decent paying job there is no way I would ever consider doing those kinds of studies again. Simply not worth it.

The problem is increasing the reward makes the people “who need the money” much more willing to engage in trials with much worse possible side effects. As long as the pay isn’t life altering money for anyone, nobody will volunteer to do something with a high probability of life altering side effects.

Exactly my thoughts. Hardly anyone from these groups is doing it “for science.” It’s for money.

Thinking otherwise is willful ignorance, and this is clearly exploitation of lower income groups.

The treatment itself is also free. For a vaccine trial that's not much of an incentive, but for things like cancer trials it can be extremely valuable.