EDIT: I actually meant smallpox when I was talking about polio. That's a big error; corrected it above. Thanks rdl.

I should have been more specific - I meant that if we did develop a vaccine, then it makes following the [smallpox] example theoretically possible, but this is still extremely challenging due to the reasons given.

Although you and I might have the vaccine, not everyone has it - not by a long shot - because not everyone is born in a modern westernized hospital. These people are protected by virtue of an extreme mobilization to literally isolate every single instance of the virus. It (hopefully) only exists in the lab at this point.