In the U.S., cases of polio are now extremely rare, and ironically, are almost always caused by the Sabin vaccine itself - being live, the virus can mutate to a stronger form.

• 在美国，小儿麻痹症病例目前极为罕见，具有讽刺意味的，几乎都是由沙宾疫苗本身-活而引起，该病毒可以发生变异，一个更强大的形式。