three types of polio virus
Type 2 polio is essentially eradicated, but the type 2 attenuated virus in the vaccine can mutate to cause polio in one out of a million cases. Most of the cases traced back to the vaccine are of type 2 virus (and I should note most of in cases were not in the kids immunized but in playmates who didn't get the vaccine, ocaretakers etc. who had no immunity to polio or had weakened immune systems due to malnutrition, cancer, pregnancy etc).
But to put it into perspective: there were 32 cases of paralysis from the vaccine last year.
so the hope is that the oral vaccine will work just as well with fewer paralysis cases.