You have likely heard that the COVID-19 vaccines are 90%, 95%, or even 100% effective. There are numerous problems with such claims.

Firstly, a definition. Leaving aside the appropriateness of the word "vaccine", Vaccine Effectiveness (VE) is the proportionate reduction of a given outcome, e.g. infection, hospitalization, death, etc.

Problems with Trials

In late 2020, Pfizer found in its initial trials that of their 44,000 volunteers split evenly between placebo and vaccine groups, there were 170 COVID cases in total. 162 were in the placebo group, versus only 8 from the vaccinated group. 8 is 5% of 162, so there is a 95% reduction in cases in the vaccine group compared to the placebo group - so it's claimed the VE (Vaccine Effectiveness) is 95%.