The World Health Organization will mount an aggressive vaccination campaign in five North African nations after two children were paralyzed by the poliovirus in Nigeria, the first such outbreak there in two years, officials said Friday.

Michel Zaffran, who directs the WHO's efforts to eradicate the disease, said a six-round campaign of vaccinations for children younger than 5 would begin shortly in Nigeria's northeastern Borno state and quickly expand to Chad, Cameroon, Niger and the Central African Republic in coming weeks.

The effort is “a major response to what we consider a major threat to the polio eradication initiative,” Zaffran told reporters.

Polio paralyzes between 1 in 200 and 1 in 1,000 infected children, meaning that the virus is probably circulating much more widely than the two cases would seem to indicate, said Jeffrey Klausner, a professor of medicine and public health at the University of California at Los Angeles.

The Associated Press reported Friday that military helicopters had rushed a polio vaccine to parts of Nigeria's northeast.

Tests showed the two affected children were sickened by the Type 1 poliovirus, one of two wild types known to remain in the world.

The new outbreak is a major setback for the eradication of polio, which had not been detected in Nigeria since July 2014. WHO will not declare a region polio-free until there are no confirmed cases for three years, so the clock must be reset.

Until this week's announcement, polio recently had been found only in Pakistan and Afghanistan, which have made progress, raising the prospect of worldwide eradication of the wild virus. Type 2 polio was declared eradicated last September and Type 3 has not been seen in more than three years, Zaffran said.