这种政策在某种程度上有计划的保护了诉讼法，也为美国做出了很大的贡献。2007年剖腹产的比率上升到了将近32%，这是这几年唯一公开的数据。
Such policies, designed in part to protect against litigation, have contributed to the rise of the U.S. cesarean rate to nearly 32% in 2007, the most recent year for which data are available.
任何特定疫苗的成功都依赖于所谓的免疫屏障，即通过高比率的免疫人群来保护不免疫的个体。
The success of any given vaccine depends on so-called herd immunity, in which a high rate of immunization in a population helps to protect those individuals who are not immune.
上周，在维也纳举行的国际艾滋病会议上，英国健康保护局发布的研究显示艾滋病毒比率在中年人群中呈增长趋势。
Research published last week at the International Aids conference in Vienna by the UK's Health Protection Agency shows that HIV rates are increasing among people of middle age.
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