Type 2 circulating vaccine-derived polioviruses (cVDPV) have caused hundreds of cases of paralytic poliomyelitis and now accounts for > 94% of polio cases since 2006. To address cVDPV and the risk of vaccine-derived polioviruses, the World Health Organization (WHO) scheduled the type 2 component of oral poliovirus vaccine (OPV) for global withdrawal and planned a synchronized switch from trivalent oral poliovirus vaccine (tOPV) to bivalent oral poliovirus vaccine (bOPV), which only types 1 and 3 attenuated polioviruses. The switch is one step in the WHO Polio Eradication and Endgame Strategic Plan 2013-2018, which describes specific steps to take to successfully achieve eradication.
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The 155 countries and territories that used OPV in immunization programs now report that they completely stopped using tOPV in May 2016. (All manufacturers of OPV ended production of tOPV before the switch.) All countries not already using inactivated polio vaccine (IPV) have committed to introducing it. As of August 2016, 173 of 194 WHO countries introduced IPV into their immunization programs—despite a global shortage of IPV.
According to the CDC, the global cooperation in stopping tOPV use has gone smoothly and is, in fact, “unprecedented.” Although this represents a milestone in the effort to eradicate polio, the CDC warns that vigilance is still needed. For example, clinicians should destroy any remaining tOPV found in a vaccine storage refrigerator or freezer. All remaining type 2 polioviruses, including type 2 wild poliovirus, type 2 vaccine-derived polioviruses, and the type 2 Sabin polioviruses used in tOPV and monovalent OPV type 2, also should be destroyed or appropriately contained in certified poliovirus-essential facilities.
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If type 2 poliovirus outbreaks occur, the United Nations Children’s Fund has a global stockpile of approximately 36 million doses of monovalent OPV type 2, with 100 million more to become available soon. Hundreds of millions of doses stored in bulk form also are available for conversion, the CDC says.
Ultimately, the CDC claims that it will not know how well the process went until it knows the number of polio cases caused by cVDPV2s that arise after the tOPV withdrawal, “with fewer cases indicating a greater success.” As of August 31, 2016, no new cVDPV outbreaks have been identified in 2016.