LONDON (AP) — Four African countries have reported new cases of polio linked to the oral vaccine, as global health numbers show there are now more children being paralyzed by viruses originating in vaccines than in the wild.

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. Seven countries elsewhere in Africa have similar outbreaks and cases have been reported in Asia, including the two countries where polio remains endemic, Afghanistan and Pakistan.

In rare cases, the live virus in oral polio vaccine can mutate into a form capable of sparking new outbreaks. All the current vaccine-derived polio cases have been sparked by a Type 2 virus contained in the vaccine. Type 2 wild virus was eliminated years ago.