它们可以产生足够多的新病毒来训练免疫系统杀死并记住它们，所以人们只需要接种一次疫苗。

They can produce enough new viruses to train the immune system to kill and remember them, so people only need to take the vaccine once.

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约两个月前，他们接受了第一次甲流疫苗接种；21天后，超过85%的志愿者对这种新型流感病毒菌株显示出了免疫力。

They got their first shot to test the vaccine about two months ago, and, after 21 days, more than 85 percent showed an immunity to this novel flu strain.

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NEJM的一项研究还显示，许多美国老年人和1976年猪流感疫苗的接种者可能都已经对这种新病毒有了免疫力。

One of the NEJM studies also showed that many older Americans as well as recipients of the 1976 swine flu vaccine may already be protected against the new virus.

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