The real meaning of ‘90% efficacy’ for Pfizer vaccine

On November 9, Pfizer and BioNTech announced that their modifed RNA vaccine against Covid-19 had a “90% effective” [1]. That is absolutely a great news in the mid of the third wave of Covid-19 pandemic. However, there have been a lot of misunderstandings of the 90% efficacy. This commentary explains the source and the meaning of that number.

It is unfortunate that the Pfizer’s data have not been peer-reviewed by independent experts or published in a medical journal. And, that is why the data are open to misinterpretation. Many people, including those in the medical community, understand that a vaccine efficacy of 90% means 9 out of 10 people got the jab will not be infected. Indeed, a commentary in the Conversation states that “The Pfizer/BioNTech vaccine reports 90% efficacy, which means that their vaccine prevented COVID-19 symptoms for 90% of volunteers that received the vaccine compared to placebo” [2]. Unfortunately, that explanation is potentially misleading.

Vaccine efficacy (VE) is defined in terms risk (i.e., probability), not the actual number of infected individuals. The standard definition of VE is provided by the CDC [3] and I rephrase as follows:

VE = 1 — (Rx / R0) ;

where Rx and R0 are the risks of infection among those in the vaccinated group and unvaccinated…