Most full scientific trials to run to conclusion, be written up in a medical journal and receive full peer review takes many years.
If a trial looks promising, and hundreds of people are dying steps can be skipped and chances taken in the hope that the new treatment works as indicated in early trials.
That is why all the vaccines are considered experimental. Since the vaccines have been used on millions of people and only a small number have had bad reactions, it was a wise choice to vaccinate.
Remember for decades a ulcer was considered a stress issue, and people were told to reduce stress and eat bland foods and take ant acids. Then a doctor found a virus was the real cause. He actually infected himself, and got an ulcer, and took the cure to prove to himself it worked. It was still a slow process to get the medical community to accept that stress was out, and it was just a virus, Helicobacter pylori.
https://en.wikipedia.org/wiki/Timeline_of_peptic_ulcer_disease_and_Helicobacter_ pylori