广播电视英语短句 第 219 课：小儿麻痹症-2

She explains that health workers have been using what are called trivalent vaccines in some places. 她解释说保健人员在一些地区已经使用称之为三价疫苗。

These are areas like Afghanistan where more than one kind of polio virus exists. 一些地区如阿富汗有超过一种类型以上的小儿麻痹病毒存在。

There are three types of polio virus. 小儿麻痹病毒有三种类型。

The trivalent vaccine is least effective against type three, more effective against type one and highly effective against type two. 三价疫苗对 预防三型病毒（ 第三种）功效很小，对 预防一型（ 第一种类型）非常有效，对 预防二型（ 第二种是）最有效的。

As a result, few new cases of type two have been reported since nineteen ninety-nine. 因此，从 1999 年起只有少数新的 二型 （第二种类型的）病例被报道。