This was not a naturally occurring attenuated virus as used in smallpox, but a virus that was attenuated in the laboratory, basically by propagating it in culture and looking for mutants that were formed as you propagated this virus under different experimental conditions.

这不是在制造天花疫苗时,使用的那种天然减毒病毒,而是一种在实验室中被人工弱化的病毒,基本上是通过在细胞中培养繁殖后,从这些培养出来的病毒中挑选出,不同实验环境下的变异体

耶鲁公开课 - 生物医学工程探索课程节选