Although the number of wild poliovirus 1 (WPV-1) cases in Nigeria has dropped significantly in 2014 with only 6 confirmed cases (compared to 53 recorded  last year), the west African country has seen 26 cases of type 2 circulating vaccine-derived poliovirus (cVDPV2) this year.

Nigeria map/CIA

During the past week, Nigeria saw five additional cVDPV2 cases, all in districts or states that had not reported cVDPV2 during 2014 to date.

According to the Global Polio Eradication Initiative, in selected high risk areas of Kano state, supplementary immunization activities (SIAs) using both inactivated polio vaccine (IPV) and oral polio vaccine (OPV) had taken place from 15 – 20 November. Large-scale Subnational Immunization Days (SNIDs) are planned for 13 – 16 December across northern Nigeria. The aim is to boost immunity to all strains of poliovirus, to rapidly interrupt circulation of both WPV1 and cVDPV2.

The GPEI says 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 vaccine-derived poliovirus.

Related: Afghanistan reports jump of 6 polio cases, South Sudan sees two new cases of cVDPV2

Elsewhere, the total WPV-1 cases in Afghanistan in 2014 is now at 20 as two new wild poliovirus type 1 (WPV1) cases were reported in the past week.

In Pakistan, Ten new wild poliovirus type 1 (WPV1) cases were reported in the past week. Six were from the Federally Administered Tribal Areas (FATA), with 1 in South Waziristan, and 5 in Khyber Agency; 1 from Killa Abdullah district in Balochistan province; 1 from Mardin district in Khyber Pakhtunkhwa province; and 2 from Sindh province (1 in Bin Qasim town of Karachi city and 1 in Badin district, southern Sindh province). The total number of WPV1 cases in Pakistan in 2014 is now 246, compared to 63 at this time last year.

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