Here's what I think it means that the new @pfizer/ @BioNTech_Group vaccine has an effectiveness of 90%.

They define effectiveness as VE=100*(1-IRR), where IRR is the infection rate ratio, i.e. the ratio of the illness rate in the vaccine group relative to the placebo group.
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Because the randomization ratio was 1:1, we know 90=100*(1-p1/p0)=100*(1-I1/I0), where p1/I1 and p0/I0 are the rate/number of infections in treatment and placebo.

Because there were 94 total cases of COVID, there must have been 9 in the vaccine group, and 85 in placebo.

With 38,955 people receiving both doses, this corresponds to an infection rate of 0.04% with the vaccine, and 0.44% without.

Here's the full trial protocol: https://www.pfizer.com/clinicalprotocol