 However, it is still possible polio circulates under these circumstances, as was the case for Nigeria, where a particular strain of virus resurfaced after five years in 2016.This can be due to chance, limited surveillance and under-vaccinated populations.Moreover, for the currently circulating virus WPV1, only 1 in 200 infection cases exhibit symptoms of polio paralysis in non-vaccinated children, and possibly even fewer in vaccinated children.