In a statement this week, WHO said two children in Sudan — one from South Darfur state and the other from Gedarif state, close to the border with Ethiopia and Eritrea — were paralyzed in March and April. Both had been recently vaccinated against polio. WHO said initial outbreak investigations show the cases are linked to an ongoing vaccine-derived outbreak in Chad that was first detected last year and is now spreading in Chad and Cameroon.

"There is local circulation in Sudan and continued sharing of transmission with Chad," the U.N. agency said, adding that genetic sequencing confirmed numerous introductions of the virus into Sudan from Chad.

WHO said it had found 11 additional vaccine-derived polio cases in Sudan and that the virus had also been identified in environmental samples.