Progress towards Polio Eradication, Nigeria, January-December 2017
http://apps.who.int/iris/bitstream/10665/260397/1/WER9309.pdfhttp://apps.who.int/iris/bitstream/10665/260397/1/WER9309.pdf
This article places special emphasis on access issues in northern Nigeria, especially Borno State, from which four WPV were confirmed in 2016.