Another vote for "pretty sure this is wrong". Think you're talking about stage 1 trials. Stage 3 calculates efficiency.
For argument sake, they give 100 people the vaccine and 100 people a placebo. If 10 people in the placebo group catch covid and only 1 person in the vaccine group catches covid it's 90% efficient? Someone cleverer than me can correct the maths, but that's the jist.
Yes, this is definitely right - I asked the doctor on the novavax trial that I and several others are on and the rate of approval essentially will depend on the amount of coronavirus knocking around. The more there is the quicker they'll be able to prove the efficacy.
Obviously they are still looking out for adverse reactions as well.
Having had no reaction whatsover I suspect I'm on the placebo, but hopefully not.