也不是所有的疫苗都是终身有效的，比如破伤风疫苗就是个很好的例子，它只能提供十年的收效保护期，过了十年，就必须要重新接种疫苗。

And not all vaccines offer long term protection. The tetanus vaccine is a good example. It offers protection for only about ten years. Then a person must be immunized again.

youdao

尼日利亚重新开始免疫接种，自去年以来已使病例减少50%。

Nigeria has restarted immunization and has already cut cases by 50% since last year.

youdao

但是，如果通过儿童免疫接种设法让重症肺炎的致死率减少一半，那么如何对付剩下的肺炎将需要重新思考。

But if childhood vaccination programmes manage to halve mortality from severe pneumonia, strategies to combat the remaining disease will require a complete re-think.

youdao