Question: We keep talking about vaccine efficacy.  I thought 95% efficacy meant that 95 of 100 people vaccinated would be protected.  Is that correct?

Answer: That’s what a lot of us think vaccine efficacy means, but alas, it’s incorrect.  Instead of thinking about vaccine efficacy in terms of “people protected,” we should think about it in terms of “risk reduction.”  Pfizer had 95% efficacy and Moderna had 94% efficacy in clinical trials.  This means that the two vaccines reduced the vaccinated population’s risk of becoming sick with COVID by 95% and 94% respectively.  

Two days ago, Lancet published a short comment on this very topic, What does 95% COVID-19 vaccine efficacy really mean?.  As the author describes, if we estimate that the virus has an attack rate of 1% over a 3 month period, a non-vaccinated population would expect to experience 1,000 COVID cases per every 100,000 individuals.  If that population were vaccinated with a 95% effective vaccine, we would expect a 95% reduction in the risk of disease; instead of 1,000 cases/100,000 individuals over 3 months, we would expect 50 cases/100,000 individuals over 3 months.

A quick note on vaccine efficacy vs. effectiveness:  Vaccine efficacy is a measure of how well the vaccine performs in clinical settings.  Meanwhile, vaccine effectiveness is a measure of how well the vaccine prevents disease in “real world” settings, e.g. outside of clinical trials.  The data from Israel shared in yesterday’s Q&A was vaccine effectiveness; preliminary analytic results show Pfizer’s vaccine effectiveness (94%) is nearly equivalent to its efficacy (95%).