The reason that the switch was made from "live" to "inactivated" is that there is a theoretical risk of contracting a mild form of polio using the live vaccine. Some children develop VAPP (vaccine associated paralytic polio) or "floppy baby" syndrome, which can have severe consequences. Because there is no circulating polio in Europe (indeed in most of the world), it was deemed by the WHO to be unethical to expose vaccinees to this theoretical risk. Hence the switch to inactivated polio, in contries where the virus has been eradicated.