Different COVID-19 vaccines are said to have x% efficacy (x ~ 85%-95%). How should we understand this? There are several ways in which we could interpret this measure:

Is it the expected percentage of people who develop antibodies? The percentage of people who do not develop any symptoms after being exposed to COVID-19?

  • A good place to start would be the manufacturers of the vaccines and the regulatory agencies that have approved them. What have you found there?
    – Armand
    Jul 4 at 11:48
  • Dupe at biology: biology.stackexchange.com/q/96941/27148 I think there is a duplicate here but I forget which. In summary, all the major trials I am aware of defined efficacy as a ratio of the number of vaccinated to unvaccinated people who get ill. Jul 4 at 23:14
  • @BryanKrause iirc, efficacy in trials was measured using criteria like death, hospitalization, serious illness, mild illness, etc.
    – Armand
    Jul 5 at 12:58
  • 1
    @Armand Those were alternative endpoints. The primary endpoints were symptomatic illness, that's it. Jul 5 at 16:22

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