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.