If you're trying to prevent a disease that only occurs at a rate of 20 per 100,000 people you have to give the vaccine to millions in order to see if the number goes down.

如果你尝试去防治一个,发病概率为十万分之二十人的疾病,那么就必须将疫苗分发给数百万人,以观察疫苗能否降低发病率

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