It could be argued that we need a safe, effective coronavirus vaccine, like, yesterday. We also know that approving new FDA-regulated medical products such as vaccines takes time. The normal approval path involves extensive testing in the lab, in animals, and then in humans for safety and efficacy. Human subject research is closely overseen to protect participants’ safety, and the gold standard for vaccine approval is a Phase 3 randomized trial, where participants get either the actual vaccine or a placebo, and then we hurry up and wait potentially months to see if there’s a difference in disease rates between the two groups.
Or we could give a bunch of people the vaccine, and then deliberately expose them to the disease to see if they get sick, like some have discussed for testing Covid-19 vaccines. This type of study is called a “challenge trial,” and we may be able to draw conclusions faster from a challenge trial than from your standard Phase 3 study. But “exposure to pathogens in challenge trials is usually permitted only for diseases that aren’t fatal or that have treatments available,” according to the Washington Post story linked above. Well, Covid-19 kinda fails both parts of that test — it can be deadly, and solidly effective treatments have yet to be identified.
The story linked above also contains links to additional discussions about challenge trials, which we encourage you to read.