Who knows, but the point is it's different in different countries.

谁知道呢,不同国家有着不同的理由

耶鲁公开课 - 金融市场课程节选

None of you got smallpox vaccinations, because once a country was certified to be smallpox free, there was no reason to-- there was no reason to vaccinate people any longer.

你们当中没人接种过天花疫苗,因为一旦一个国家确认不再有天花,就没有理由,就没有理由再给人们接种

耶鲁公开课 - 生物医学工程探索课程节选