Last week was supposed to mark two years since Nigeria had seen a case of wild poliovirus, leaving Afghanistan and Pakistan as the only two countries left in the world where the virus was still circulating. Instead, on Thursday (August 11), the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC) released statements noting that the Nigerian polio surveillance system had documented two children in the northern Borno state who have been paralyzed following poliovirus infection.

“It’s a blow,” Sona Bari, a spokeswoman for the WHO’s polio program, told STAT News. “It’s the first time in history that a country has stopped transmission and then found indigenous virus again.”

Genetic sequencing of the viruses responsible for the two cases revealed that both are related to the last wild poliovirus strain detected in Borno in 2011. This suggests that the virus may have circulated...

In response to the new cases, the WHO and other partners of the Global Polio Eradication Initiative are helping the Nigerian government to strengthen surveillance in the country and conduct large-scale immunization campaigns in Borno and neighboring states. Officials hope to vaccinate as many as 1 million children in the next few days, Stephen Cochi, a senior polio program scientist at the CDC, told STAT.

“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,” Matshidiso Moeti, WHO regional director for Africa, said in the agency’s statement.

The last case of wild poliovirus reported in Nigeria was documented in July 2014. The WHO will not declare a region polio-free until there have been no confirmed cases for three years. The new cases in a region that was nearing that mark reflect the political and social challenges—namely, raids by the Islamic fundamentalist militia Boko Haram and battles between the group and the Nigerian Army, which have limited access to certain regions and has forced thousands of residents from their homes—in vaccinating all children in the area.

“That fluid movement of population complicates understanding of exactly where they’ve ended up,” John Vertefeuille, director of polio eradication for the CDC, told The New York Times. “This is a setback, but we need to double our effort to make sure we interrupt transmission.”

Update (October 7): The WHO issued a statement announcing three laboratory-confirmed cases of wild poliovirus type 1 (WPV1) in Borno State, Nigeria. Two of the positive tests corresponded to samples from two paralyzed children. In the case of a third paralyzed child, poliovirus couldn’t not be isolated, said Oliver Rosenbauer, a spokesperson for the WHO’s Global Polio Eradication Initiative. So public health workers “checked a close healthy contact,” who tested positive for the virus, Rosenbauer told The Scientist. “Obviously this is tragic news, and a setback because everyone had assumed Nigeria (and Africa) is polio-free.”

In addition, strengthened disease surveillance activities turned up one case of vaccine-derived virus in a healthy child. “There have been no cases of paralysis identified with this strain so far, but we know that this strain is circulating and is therefore also considered an outbreak which must be addressed,” Rosenbauer said.

“The overriding priority now is to fully implement the outbreak response being conducted not just in Nigeria but across the region, including in the neighboring countries of the Lake Chad basin, to try to ensure that no further children will be affected by this outbreak.”

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