Three Years with No Wild Poliovirus Type 3

With no sign of type three of the virus anywhere in the world for three years, it’s very likely that it has been eradicated. The last case was reported in Yobe in northern Nigeria, when an 11 month old boy was paralysed on 10 November 2012. While three years without the virus is not a sure sign that it has been eradicated, it is extremely encouraging news for the global effort to eradicate all strains of this disease.

This year we have already seen remarkable progress, with type 2 WPV being declared eradicated by the Global Certification Commission in September. They declared the virus stopped, with no reported case since 1999.

The only wild poliovirus type which has been reported since the last case in Nigeria in 2012 is WPV1. This demonstrates significant progress and enables the polio eradication programme to zero in on this last remaining serotype, committing all resources to fighting type 1.

The GCC gathered to declare the eradication of WPV2 because it was a precondition for the switch from trivalent to bivalent oral polio vaccine which will take place in April 2016. It is the first phase of the removal of all oral polio vaccines, needed to eliminate the small risks of vaccine-derived polioviruses.   Find out more about the role of certification, surveillance and containment in stopping a global disease.

Strong surveillance is the key to giving experts the confidence that any type of wild poliovirus has been stopped. Type three of the virus is less virulent than type one, causing cases at a rate of approximately one in 1,000 infections compared with one in 200. While causing fewer cases is a good thing, it also means the virus can transmit silently for longer without being detected.

This makes sensitive surveillance all the more important.

Continuing to search for and trace each virus over time is the strongest guarantee we can have that the virus is not circulating without showing itself through cases of paralysis. It is essential that strong surveillance is continued in all countries, not just those where poliovirus continues to circulate.

Until 2009, all three types of polio were targeted using three tools: trivalent oral polio vaccine (tOPV) which protects children against all 3 strains however with many needed doses, and monovalent OPVs type 1 and 3 which protect children with fewer doses, but only against those specific serotypes. Yet while type 2 hadn’t been seen since 1999, types 1 and 3 continued to circulate and paralyse children, causing alternate outbreaks year on year.

The new decade saw in a game-changer. Bivalent oral polio vaccine, which vaccinated children against types 1 and 3 simultaneously and more effectively than trivalent OPV, had been developed and was licenced in record time. With the removal of the type 2 component, the level of immunity against the other two strains was much stronger, and cases of type 3 rapidly declined, as shown in the graph below.

The Global Polio Eradication Initiative’s ability to innovate through the development and delivery of a new vaccine, so close to the endpoint of the programme, was a remarkable achievement that was key in stopping WPV3.

While this milestone is not a guarantee that WPV3 is gone, three years without the virus is a historic achievement and turns the focus of the world all the more sharply on stopping wild poliovirus type 1.