More polio cases now caused by vaccine than by wild virus

In rare cases, the live virus in oral polio vaccine can mutate into a form capable of sparking new outbreaks.

Western countries use a more expensive injectable polio vaccine that contains an inactivated virus incapable of causing polio.

In a report late last week, the World Health Organization and partners noted nine new polio cases caused by the vaccine in Nigeria, Congo, Central African Republic and Angola. (ABC27)