After more than two years without the detection of wild polio in Nigeria, the Government reported three laboratory confirmed wild poliovirus type one (WPV1) cases with onset between July and August 2016.
All three cases were detected from Borno State, in children between 2 and 5 years of age. Two of them developed acute flaccid paralysis (AFP) while one was asymptomatic, identified as a close contact of a child with AFP.
The detected viruses are closely linked to WPV1 last detected in Borno in 2011. This is an indication that this virus strain has been circulating without detection since that time.
In addition to the wild polio cases reported, a vaccine-derived poliovirus type 2 (cVDPV2) has also been detected in specimens collected from a healthy household contact of one of the WPV1 cases recently reported as part of strengthened disease surveillance activities being implemented in the area.
The genetic analysis of the isolated strain indicates that also this cVDPV2 strain has been circulating in the area for at least two years without prior detection.
What is a vaccine-derived poliovirus?
The Global Polio Eradication Initiative 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.
A regional outbreak response in north-eastern Nigeria continues to be implemented, in response to the detected wild and vaccine derived polio virus.
Large-scale supplementary immunization activities are currently being implemented. In response to detection of polio in Borno, the government of Nigeria has declared the outbreak to be a national public health emergency; and the neighbouring governments of Cameroon, Central African Republic, Chad and Niger declared a regional public health emergency for the Lake Chad sub-region.
The detection of wild poliovirus type one (WPV1) and vaccine derived poliovirus type 2 (cVDPV2) in Nigeria underscores the importance of maintaining high levels of routine vaccination coverage at all levels to minimize the risk and consequences of poliovirus circulation.