Ann: Phase III prophylaxis trial for CS-8958 , page-22

I think it is somewhere along this line.

Say the group that take Inavir, 2/10 people get sick.
The other group that take placebo, 4/10 people get sick.
Efficacy % is a comparation of these 2 groups, hence even thought it is 80% (inavir efficiency) by itself, it is not right to just look at 1 group only; you need to compare them.
By comparison, may be 2:4 (2 against 4) = 50% efficacy.
Hence, if more people get sick in the placebo group, it will up the efficacy %. i.e. if 8/10 in placebo get sick, it is 2:8 = 75% efficacy.

just guessing here.