这种疫苗对流感的免疫效力时间较长。

The vaccine provides longer immunity against flu.

《牛津词典》

由于这些新疫苗可能在流感大流行开始之后才能够获得，因而要评估它们的安全性和效力就很可能非常困难。

Evaluation of safety and efficacy of new vaccines is likely to be especially challenging since the vaccines might not be available until after the pandemic had started.

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出于这些理由，世卫组织建议所有接种大流行性流感疫苗的国家对安全性和效力进行严密监测。目前许多国家已为此制定计划。

For these reasons, WHO advises all countries administering pandemic vaccines to conduct intensive monitoring for safety and efficacy, and many countries have plans in place for doing so.

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So,as the vaccine was passed from arm to arm, it became less potent, and the length of protection that it would give you varied.

所以当疫苗从一个人传给另一个人之后,它的防治效力会减弱,它带给人的免疫期限也会改变

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