The virus did not mutate to a more virulent form.

VOA: standard.2010.01.18

In the Sabin vaccine, or the oral polio vaccine, now you have a non-virulent virus.

在沙宾疫苗,即口服小儿麻痹症疫苗中,存在非致病性病毒

耶鲁公开课 - 生物医学工程探索课程节选

In the Sabin vaccine, or the oral polio vaccine, now you have a non-virulent virus.

在沙宾疫苗,即口服小儿麻痹症疫苗中,存在非致病性病毒

耶鲁公开课 - 生物医学工程探索课程节选