有人能请告诉我，为什么疫苗公司测试新疫苗用于新适应症时，不用安慰剂，甚至不使用佐剂盐水作为对照，而是用针对不同疾病的旧疫苗来生成安全性数据？

这种设计中安慰剂对照在哪里？这怎么算道德？

Can someone PLEASE tell me why vaccine companies get to test new jabs for new indications not against placebo, or even adjuvanted saline, but against older shots for different diseases to generate safety data?

Where is the placebo control in this design? How is this ethical?