It's not even really that the live version mutated to become transmissible, exactly - the live oral polio vaccine is reasonably transmissible through the usual

It's not even really that the live version mutated to become transmissible, exactly - the live oral polio vaccine is reasonably transmissible through the usual fecal-oral route full stop. The problem is that as it spreads through a community the vaccine virus - and especially the type 2 poliovirus - tends to mutate in ways that undo the attenuation which allows it to safely be used as a vaccine and cause it to spread, paralyze and kill in the same way that wild-type polio virus does. Viruses with these kind of mutations have been circulating in the developing world for some time and there seems to be concern they may now be in the UK. (In fact this is the only kind of type 2 poliovirus in circulation these days - the wild version of it was eradicated a few years ago. There were a bunch of celebratory announcements of this that made it to HN which were probably rather premature. The WHO and their partners had a fancy endgame plan for this involving pulling the type 2 component of the vaccine, at which point it would go extinct for real and we'd no longer have to deal with it. This plan failed spectacularly.)