Volunteers will not be exposed to the SARS-CoV-2 virus as part of the study. These are studies of preventive vaccines that we hope will keep people healthy.
If you become infected with SARS-CoV-2 and become ill with COVID-19, the study staff will work with you to make sure you get the care that you need.
The answer to which could be as simple as "they were given the placebo"
Presumably if you're uninsured, they get you medical insurance. Testing drugs on uninsured people then leaving them untreated if they get ill during the study would be a bit cold, even for the american medical industry.
If they do the trial in large enough numbers, many participants will get exposes in their normal lives anyway. I assume they will target hotspot areas.
That is a thing that people do sometimes:
https://en.wikipedia.org/wiki/Human_challenge_study
There has been a debate about also doing it with COVID-19:
https://en.wikipedia.org/wiki/COVID-19_vaccine#Proposed_chal...