Circulating vaccine-derived poliovirus outbreaks in 5 countries, 2014–2015

In 2015, wild poliovirus (WPV) transmission was identified only in Afghanistan and Pakistan. The widespread use of live, attenuated oral poliovirus vaccine (OPV) has been crucial in polio eradication efforts. However, OPV use, particularly in areas with low vaccination coverage,

is associated with a very low risk of emergence of vaccine-derived polioviruses (VDPV), which can cause paralysis.

VDPVs differ genetically from vaccine viruses and can cause outbreaks in areas with low vaccination coverage. Circulating VDPVs (cVDPVs) are VDPVs in confirmed outbreaks. Single VDPVs for which the origin cannot be determined are classified as ambiguous or aVDPVs, which can also cause paralysis. Among the 3 types of WPV, type 2 has been declared by WHO to be

eradicated. More than 90% of cVDPV cases have been caused by type 2 cVDPVs (cVDPV2). Therefore, in April 2016, all OPV-using countries of the world will discontinue the use of type 2 Sabin vaccine by simultaneously

switching from trivalent OPV (types 1, 2 and 3) to bivalent OPV (types 1 and 3) for routine and supplementary immunization.