Sparked by concerns about a surge of poliomyelitis cases in Nigeria caused by a vaccine-derived strain of type 2 poliovirus, health officials stepped up efforts to curb its spread, according to the Global Polio Vaccine Initiative (http://www.polioeradication.org/content/general/current_monthly_sitrep.asp).
Although type 2 wild poliovirus (1 of the 3 serotypes of wild poliovirus) had been eradicated in 1999, it reemerged a few years ago when weakened type 2 virus in oral polio vaccine mutated. For reasons not well understood, circulating vaccine-derived poliovirus began spreading more aggressively in 2009. By late July, 124 cases of paralysis caused by vaccine-derived virus had been confirmed for 2009, more than 4 times the number of cases confirmed in July 2008.
Stephenson J. Polio Surge. JAMA. 2009;302(11):1161. doi:10.1001/jama.2009.1337