来自《耶鲁大学开放课程：生物医学工程探索 第9集 第9集》

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  the kind of response that you get could change
  它的效果会发生改变
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  because the virus itself mutates
  这是因为病毒自身变异
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  or because you're also passing on some factors
  或者这个过程也传播了一些东西
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  that provide some protection
  它提供了某种保护机制
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  that changes the course of disease in the recipient.
  改变了疾病在疫苗接受者体内的发病过程
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  So,as the vaccine was passed from arm to arm,
  所以当疫苗从一个人传给另一个人之后
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  it became less potent,
  它的防治效力会减弱
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  and the length of protection that it would give you varied.
  它带给人的免疫期限也会改变
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  So, if you were someone that was getting it
  如果一个疫苗经过很多人的传递
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  after it had been passed through many people,
  再接种到你身上
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  you wouldn't have as strong or as lengthy a protection
  你不会获得像第一个接受疫苗的人
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  as the first individual did.
  那样强效持久的免疫效果