from John Dee’s Almanac
What Does 90% or 95% Effective Mean?
When vaccine manufacturers via the media and/or the voice of Her Majesty’s Government claim a vaccine is “90% effective” (or even “95% effective” in the case of Pfizer’s product) please do bear in mind that this an estimate of relative risk as opposed to absolute risk, and can be somewhat misleading for the general public who will interpret this as “if I have the vaccine I’m 90%/95% certain of not getting COVID” – this is not what “90%/95% effective” means.
Pfizer’s famous “95% effective” claim is, in fact, a measure of relative risk. A total of 8 of their vaccinated group of 18,198 participants went on to develop COVID-19 (0.044% risk), whereas a total of 162 of their un-vaccinated group of 18,325 participants went on to develop COVID-19 (0.88% risk). The relative risk reduction calculation is thus 100 x 1-(0.044/0.88) = 95%, whereas the absolute risk reduction is a mere 0.88 – 0.044 = 0.84%.
Thus, if you were a vaccinated subject in Pfizer’s trial, there was a 99.96% chance you didn’t develop COVID-19.
If you were an un-vaccinated subject there was still a 99.12% chance you didn’t develop COVID-19, the difference being the absolute risk of 0.84% i.e. vaccination by Pfizer’s product actually reduces your personal risk by less than 1% and not 95%!
This, of course, assumes equal risk of exposure to the virus by both groups of participants but this is merely a convenient assumption – no clinical trial can ever hope to measure thus.