It turned out that one of--there were three vaccine manufacturers, one of them was using the procedure not quite correctly, they weren't completely killing the virus when they produced their vaccine.

结果显示,疫苗的生产厂家一共有三个,其中一家的生产程序不是非常正确,他们在生产疫苗的时候,并没有完全杀死病毒

耶鲁公开课 - 生物医学工程探索课程节选