The H1N1 pandemic was characterized by the emergence of a new influenza virus to which many people had no pre-existingimmunity.
H1N 1流感大流行的特点是出现新的流感病毒，而许多人对其并没有预先存在的免疫力。
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When people have no pre-existingimmunity 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.
Whether this reflects the persistence of pre-existingimmunity at the time of revaccination or lower rates of revaccination in the study populations cannot be determined.