Nigeria's removal from WHO's list of polio-endemic countries in October had meant the entire African continent was free of the crippling disease.
Two children have been paralyzed by polio in northeastern Borno state in two different local government areas that had been cut off by Boko Haram's Islamic extremist uprising, Health Minister Isaac Adewole said in a statement Thursday night.
"Our overriding priority right now is to rapidly boost immunity in the affected areas to ensure that no more children are affected by this terrible disease," he said.
He ordered the deployment of a national emergency response team. WHO said it was working with the government to urgently prevent more children from being paralyzed, with large-scale immunizations and other measures.
It was unclear how accessible the two areas are. The United Nations last month suspended aid to newly liberated but still dangerous areas of Borno after Boko Haram ambushed a humanitarian convoy, wounding three civilians including a UNICEF worker. That came even as aid groups declared half a million people are starving in those areas and children are dying daily of starvation.
Because of the Islamic uprising in the northeast, health workers have been testing sewage and stool samples of refugees from areas too dangerous to access.
Nigeria's fight against polio has been dramatic. Two decades ago, it was recording 1,000 polio cases a year, the highest in the world.
The Islamic extremists opposed the anti-polio campaign. Boko Haram gunmen killed nine women vaccinators in northern Kano state in February 2013, but the vaccinations continued.
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