It’s been several months since any new cases of type 2 circulating vaccine-derived poliovirus (cVDPV2) have been reported in the Democratic Republic of Congo (DRC); however, the Global Polio Eradication Initiative (GPEI) reported this week about two additional cases.

DRC map
Democratic Republic of the Congo/CIA

The two new cases–one in Manono district, and one in Ankoro district, Tanganika province had onset dates of Nov. 9 and Nov. 17, respectively. This brings the total cVDPV2 cases in DRC this year to 12.

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According to GPEI, Vaccine-derived polioviruses (VDPVs) are rare strains of poliovirus that have genetically mutated from the strain contained in the oral polio vaccine. The oral polio vaccine contains a live, attenuated(weakened) vaccine-virus. When a child is vaccinated,the weakened vaccine-virus replicates in the intestine and enters into the bloodstream, triggering a protective immune response in the child. Like wild poliovirus, the child excretes the vaccine-virus for a period of six to eight weeks. Importantly, as it is excreted, some of the vaccine-virus may no longer be the same as the original vaccine-virus as it has genetically altered during replication. This is called a VDPV.

On very rare occasions, if a population is seriously under-immunized, there are enough susceptible children for the excreted vaccine-derived polioviruses to begin circulating in the community. If the vaccine-virus is able to circulate for a prolonged period of time uninterrupted, it can mutate and, over the course of 12-18 months, reacquire neurovirulence. These viruses are called circulating vaccine-derived polioviruses (cVDPV).

In addition, this week GPEI reported on an additional new case of wild poliovirus type 1 (WPV1) in Pakistan, in Kila Abdullah district, Balochistan province, with onset of paralysis on 13 November. The total number of WPV1 cases in Pakistan in 2017 is now 8 and 20 total WPV1 cases total for 2017.

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