If exposing a few people on purpose is a moral wrong, then surely allowing millions of them to contract it by accident so that a few people in your RCT can become infected by chance is a moral abomination.
FWIW, ethicists were split, but mostly on board with HCTs. It was mostly the government that was opposed.
Essentially, no one signing up for the trial would have been properly able to give consent, because no one knew the real consequences.
https://www.statnews.com/2020/06/23/challenge-trials-live-co...
Our current testing protocol requires letting the virus rampage through the population before the trials will achieve statistical significance, which is surely more unethical than deliberately exposing a small number of people.
Suppose we replace Covid with "car crashes" and vaccine with "seatbelts". This is a good analogy because crashes are rare and we can't cure trauma (like on Star Trek). It would be absurd to deliberately cause car crashes in order to study the effect of seatbelts. For the experimental group, there will still be some minor injuries. For the control group, many will die.
Phase III trials for each of these new vaccines end long after the efficacy is determined. Estimated primary completion dates are in June 2021 (BioNTech/Pfizer) and November 2021 (Moderna).
Even after the drug is accepted, monitoring for safety continues.