Update on vaccine-derived polioviruses--worldwide, April 2011-June 2012.

In 1988, the World Health Assembly resolved to eradicate poliomyelitis worldwide. One of the main tools used in polio eradication efforts has been the live, attenuated oral poliovirus vaccine (OPV). This inexpensive vaccine is administered easily by mouth, makes recent recipients resistant to infection by wild polioviruses (WPVs), and provides long-term protection against paralytic disease through durable humoral immunity. Nonetheless, rare cases of vaccine-associated paralytic poliomyelitis can occur both among immunologically normal OPV recipients and their contacts and among persons who are immunodeficient. In addition, vaccine-derived polioviruses (VDPVs) can emerge to cause polio outbreaks in areas with low OPV coverage and can replicate for years in persons who are immunodeficient. This report updates previous surveillance summaries and describes VDPVs detected worldwide during April 2011-June 2012. In 2011, a new outbreak of circulating VDPVs (cVDPVs) was identified in Yemen; a second VDPV isolate, related to a previously reported VDPV isolate, signaled an outbreak in Mozambique; and VDPV circulation reemerged in Madagascar. An outbreak that began in Somalia in 2008 continued until December 2011. Outbreaks in Nigeria and the Democratic Republic of the Congo (DRC) identified in 2005 and 2008, respectively, continued in 2012. Niger experienced a new cVDPV importation from Nigeria in 2011. Twelve newly identified persons in six middle-income countries were found to excrete immunodeficiency-associated VDPVs (iVDPVs), and VDPVs were found among healthy persons and environmental samples in 13 countries. To prevent VDPV emergence and spread, all countries should maintain high vaccination coverage against all three poliovirus serotypes; OPV use will be discontinued worldwide once all WPV transmission is interrupted.