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This is a good question that was picked up by a blog I read. IIRC it refers to the ratio of infections in the control group vs. the vaccinated group. Hence an effectiveness of 90% means 90% of cases occurred in the non-vaccinated group.

It does not mean 10% of people end up with no protection at all, as was my first thought.

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Minor nit. The ratio you refer to is called efficacy, not effectiveness. Effectiveness is the number you would get by observing people in the real world rather than the phase 3 trial. It's worth noting the difference because we're also starting to get data on effectiveness, like the CDC study that tracked healthcare workers and found the vaccine also prevented asymptomatic infection.

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