Several years ago, researchers tested an experimental AIDS vaccine in Thailand.

几年前，研究人员在泰国测试了一种实验性艾滋病疫苗。

But experimental AIDS vaccines fail at this because the virus is always mutating, or changing, to avoid destruction.

但是实验性艾滋病疫苗在这方面失败了， 因为病毒总是在变异或改变， 以避免被破坏。