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It seems like a stupid question: does it "just" mean that 90% of the population won't be infected? But if only 10% of the population would have been infected without the vaccine, then it's not effective.

Thus, does "90% effective" mean that only 1% (10% of 10%) will now get infected?

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    See also biology.stackexchange.com/questions/96941 at Biology.SE. It's more like your second paragraph, but not exactly. It's calculated based on a ratio of people who are vaccinated that get sick vs people in vaccine trials who get a placebo. Jan 7 at 23:35
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    @BryanKrause the accepted biology.SE answer does answer my question: "this ratio would tell you that a vaccine efficacy of 95% means that if you took 20 people who would have had a positive test after the placebo, you would only expect 1 of them to test positive if they instead got the vaccine."
    – RonJohn
    Jan 7 at 23:49