Wild poliovirus type 2 was last seen in 1999 and is considered eradicated. The last case of  wild polio virus type 3 (WPV3) was reported almost exactly two years ago in Nigeria.

Poliovirus  Image/CDC
Poliovirus Image/CDC

All cases due to wild poliovirus in the past two years have been caused by wild poliovirus type 1 (WPV1).

Despite this promising news, the Global Polio Eradication Initiative (GPEI) says this is no guarantee of its eradication.

One of the key dangers with WPV3 is that it is less virulent than WPV1, causing cases at a rate of approximately 1 in 1,000 infections (compared with 1 in 200 infections with WPV1). While causing fewer cases is a good thing, it also means the virus can transmit silently for longer without being detected.

The GPEI says this is still very encouraging news. It is a reminder that the eradication of polio is achievable, and that children everywhere will one day be free from the threat of this disease.

The island country of Madagascar reported a confirmed case circulating vaccine-derived poliovirus type 1 (cVDPV1) that resulted in acute flaccid paralysis (AFP). It is estimated that a quarter of  children remain under-immunized against polio.

Madagascar was previously affected by a cVDPV2 outbreak in 2001/2002 (resulting in five cases) and in 2005 (resulting in three cases).

The Global Polio Eradication Initiative (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.