They do fast track drugs if studies show overwhelming success, and it is no longer ethical to keep it from the control group.
For example 780,000 people died of polio between 1988 and today. Yet for all that time there has been a cheap, low risk, well tested, near 100% effective vaccine (many in fact). And polio is probably one of the better cases because governments and charities have been pushing it pretty hard.
Edit: Also, polio eradication efforts where a Manhattan project style heroic effort.