Polio is one of the very strange ones. It gets eradicated constantly and keeps coming back. It has odd and not consistent symptoms. Many countries do not use this shot yet have no trouble. This is also the shot that created a secondary illness - not clear on this as my research to this issue was several years ago.

However, there is something up with polio and I would look into it further if I had a kid and was required to give them this one. It's not required in SE Asia and I have not noticed anything about it being a problem here. Maybe they hide their sick people.