[loeg]

The larger the set the faster the results, but a large set is expensive. "Afterwards" is sort of a simplification. The study arms are compared regularly throughout in case the study needs to be ended one way or the other (Data and Safety Monitoring Committee). (If the vaccine is effective: great, we want to ramp up production and give it to everyone ASAP. If the vaccine is ineffective or has very bad side effects, we also want to conclude the study.)