Re: 95% Efficacy
It's more to do with how they get the figure 95%.
I'll use the proportions they got and create a different sample size to keep the numbers easier to follow.
Take one group (A) of 10000 people and vaccinate them.
Take another group (B) of 10000 and give them a placebo.
Wait for a while and then see what happens (ideally, you'd like to inject the virus into all 20000 people, but this clearly can't be done due to you likely killing people, so you'll just have to rely on "normal" exposure).
4 people in group A tested positive.
74 people in group B tested positive.
So, the vaccine "prevented" 70 people from catching the virus in group A.
70 out of 74 = 95%.
That's how they get their efficacy rate.