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.
当人们对流感病毒没有预先免疫性，他们就需要几周间隔的两次注射而不是一次注射-这在当时免疫接种很多人去保护他们操作是非常困难的。
When people have no pre-existing immunity to a flu virus, they need not just one but two doses of vaccine several weeks apart - making it very hard to immunise many people in time to protect them.
据估计，一次大规模免疫接种运动，如果得到及时实施，可预防70%的病例。
It is estimated that a mass immunization campaign, promptly implemented, can prevent 70% of cases.
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