Radiolab did a good piece on Challenge trials for the COVID-19 vaccine which you can find here.
Challenge trials are an expedient, but fraught way to generate data on a vaccine’s efficacy quickly. Briefly, they involve taking a (relatively) small group of people, say 100, and dividing them into two groups of 50 each. One group gets the vaccine and the other group gets the placebo. Then, after waiting some period of time for the immune system to generate a protective response, you expose all 100 people to live virus.
This is essentially the same thing as what is done in the more traditional trial, with the exception of directly exposing subjects.
In the end, it’s a simple matter to calculate the efficacy (or not) of the vaccine by comparing the number of people who got sick in each group. If these numbers are equal, the vaccine is ineffective, etc. Even if all 100 people got sick from this challenge, it would still be less than the number of people who got sick (170) during the Pfizer trial that involved 43,000+ total participants (reported here).
Further, a challenge trial can be completed in as little as two months, while more traditional trials are ongoing over two years.
In the Radiolab piece, trial participants were asked why they participated. Answers ranged from appeals to mathematics (one participant said exposure to COVID did not significantly increase his risk of dying in any given year) to a need to make their experience meaningful (if they caught covid in the trial, at least it would provide important evidence toward approving a drug, whereas if they caught it any other time, it would just be a waste.)
However, the one reason I did not hear was one of financial inducement. Although I could not find actual data on this, challenge trials may pay up to $4000 for the 3-6 weeks of legal quarantine. There are a number of issues associated with these payments – the most obvious being that it may induce those without means to participate because they feel the money if too good to pass up. Jennifer Blumenthal-Barby and Peter Ubel write that, in fact, the payment for these trials may not be enough as it fails to meet the proposed minimum living wage of $15/hr.
I recommend that you listen to the Radiolab piece (it’s less than 30 minutes long) and take a look at the Blumenthal-Barby and Ubel, which is an interesting read.