The H1N1 pandemic was characterized by the emergence of a new influenza virus to which many people had no pre-existing immunity.

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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Whether this reflects the persistence of pre-existing immunity at the time of revaccination or lower rates of revaccination in the study populations cannot be determined.

这是因为再次接种时前次的免疫性仍然存在，抑或是因为在研究人群中再次接种率较低，对此目前尚无定论。

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This is an important distinction because the efficacy of a live priming vaccine could be affected by existing immunity to the microorganism and to related species.

区分这个很重要，因为初次免疫的功效会受到已经存在的针对该细菌或者相关细菌的免疫性的影响。

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But the current swine flu is a new strain to which humans have no immunity and against which existing vaccines offer no protection.

VOA: standard.2009.04.30

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The current recommendation is based on unique conditions associated with the existing pandemic, including low levels of population immunity to 2009 H1N1, the potential for health-care personnel to be exposed to H1N1 patients, and other factors, Dimond says.

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