Comment Re:Sorry, I don't think I was clear (Score 2) 155
In the phase 3 studies these rates come from, effectiveness of the vaccine is computed by comparing the infection rate of the experimental group (vaccinated) vs that of the control group (unvaccinated; placebo). A 96.5% rating, for example, would mean that the vaccinated group got sick at about 3.5% the rate of the unvaccinated group. This is over a period of time -- generally until some previously determined number of total infections are recorded among all study subjects. So in all, the effectiveness is closer to "over all exposure events", though naturally only a finite number of exposure events might occur while the study is running.
FWIW In real life studies the vaccines are even more effective against preventing "severe" disease.