Since this is would presumably be testing for efficacy I guess they would have a number of pre-determined "interim analysis" points in the study when X count of people report becoming positive for the coronavirus. If the proportion infected in the treatment vs control is too high (i.e. lots of vaccinated ppl got sick) they will stop the trial and consider the vaccine a failure. If too few get it, they might stop the trial and immediately move to a Phase III study to confirm their result - that is they accelerate the trial to get the treatment out quicker.

I am unsure if they will have some lower bound for acceleration for this vaccine, since the downsides might not be apparent for a long time.