August 11, 2016, was supposed to be a historic day for the global effort to eradicate the polio virus. This Thursday would have marked two years since someone on the African continent last contracted the wild-polio virus.

If the continent made it without a case for a third year, the World Health Organization could have certified it as polio-free, and Africa would have joined four other WHO regions—the Americas, Europe, the Western Pacific, and South-East Asia—in eradicating wild polio.

But it is not to be. Nigerian health officials announced on Thursday, the same day they expected to celebrate the anniversary, that two children have been paralyzed by wild polio in the country’s northeastern Borno State.

“It has set us back,” said Isaac Adewole, the Nigerian health minister, according to the Nigerian newspaper Leadership. Borno State will now undergo three rounds of emergency immunizations, he added. Neighboring countries have also been put on alert to look for cases of wild polio.

Matshidiso Moeti, the regional director of WHO in Africa, said she was “deeply saddened” by the news. “The [Nigerian] government has made significant strides to stop this paralyzing disease in recent years. The overriding priority now is to rapidly immunize all children around the affected area and ensure that no other children succumb to this terrible disease.”