translation.en: The second challenge we had was -- in contrast to the smallpox vaccine, which was very stable, and a single dose protected you for life -- the polio vaccine is incredibly fragile.
translation.zh: 我们面临的第二个挑战是-- 脊髓灰质炎疫苗与天花疫苗性质不同 天花疫苗性质稳定，打一次就终身免疫 而脊髓灰质炎疫苗是否脆弱