词典释义：
双语例句
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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.
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。