Except failed vaccines can induce a response that makes you more susceptible to the disease and with worse symptoms. It’s called antibody-dependent enhancement:
I said that there’s plenty of incidental exposure already: healthcare workers, yes, but also nearly everyone who’s leaving the home too. It is literally a pandemic!
This point is actually made in the article itself: they want to get it done while and where there’s community transmission to avoid the ethical fraught ness of a challenge trial.
For similar reasons, challenge trials also weren’t done for AIDS PrEP, since there were groups with enough lifestyle exposure to power a study.
https://pubmed.ncbi.nlm.nih.gov/12725690/
Just about the last people you’d want to risk that kind of vaccine trial on are healthcare workers, who are already in short supply.