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We see figures like "ACMECorp covid vaccine is 90% effective". I guess that this means something like:

In the trial the number of people receiving the vaccine who caught covid is only 10% of the number who didn't receive the vaccine and caught covid.

My question is about the interpretation of the figure "90%". For example it could mean:

  • 90% of people who received the vaccine are completely protected and will never catch covid.

  • all the people who received the vaccine are 90% protected i.e. will only catch covid once out of every ten exposures to it.

  • something in between.

  • the trial doesn't tell us this so we don't know.

Do we know which, if any, of the above apply to the effectiveness figures given by Astra Zeneca, Pfizer, etc? Or is the situation (even) more complicated than I've described above?

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