Phase III tests for both safety and efficacy. The efficacy portion is the bottleneck because the experimenters do not a priori know the rate of infection. So ra

Phase III tests for both safety and efficacy. The efficacy portion is the bottleneck because the experimenters do not a priori know the rate of infection. So rather than waiting for a fixed amount of time, we must wait for a target number of infections. In contrast, since we know when each person in the treatment group got the vaccine, safety can be measured after a predetermined amount of time.