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Written by Royal Melbourne Hospital’s Associate Professor Bruce Thorley, Head of National Enterovirus Reference Laboratory & WHO Polio Regional Reference Laboratory at the Doherty Institute

In a significant step forward for the global polio eradication initiative, the World Health Organization African Region has been declared wild-polio free after four years without a case.

Adding to this significance, is that Africa was able to do this despite areas of social and political unrest not being accessible for vaccination and surveillance – because of this, WHO required the region to be polio-free for four years rather than the standard three.

In 1988, when World Health Organization (WHO) Global Polio Eradication Initiative was established, more than 1,000 cases of polio were reported each day.

So far in 2020, there has been just 104 cases of wild poliovirus.

Five out of six WHO regions around the world are now polio-free: Americas (certified polio-free in 1994), Western Pacific which includes Australia (2000), Europe (2002), South-East Asia (2014) and now, Africa (2020).

The remaining region endemic for transmission of wild poliovirus is the Eastern Mediterranean Region, with cases restricted to Afghanistan and Pakistan. Unfortunately, these two countries still face significant challenges to completing wild poliovirus eradication with the number of cases not diminishing in recent years.

No wild polio cases have been imported from Afghanistan and Pakistan since it was declared a Public Health Emergency of International Concern in 2014 – this requires travellers to and from these countries to be vaccinated for polio.

Poliovirus only infects humans, and is mainly spread by faecal-oral transmission. Adherence to basic hygiene such as washing hands and access to clean water and sewage infrastructure are important to restricting transmission.

Most people infected with poliovirus don’t have any symptoms, with less than one per cent of infections resulting in paralysis, with such debilitating consequences that any polio case requires immediate investigation. Polio vaccination prevents paralysis and reduces the amount of virus shed in the faeces.

The key to a polio-free world is vaccination, which wasn’t available in Australia until 1956. It is important that high rates of polio vaccination (>90 per cent) are maintained in all countries to stop transmission of wild poliovirus in the endemic countries and to protect the population of polio-free countries from importations of wild poliovirus or when travelling.

Until the global eradication of wild poliovirus has been certified, all countries remain at risk of a wild poliovirus importation from one of these countries as last occurred in Australia in 2007.

All forms of poliovirus (wild and vaccine strains) undergo slight changes to their genetic sequence as a normal part of virus replication. This can become an issue in areas with low rates of oral polio vaccine (OPV) coverage, where there is a risk of the live, attenuated or weakened virus being transmitted among the unimmunised population.

If the OPV strain is transmitted for a long time in an under-immunised community, an outbreak of vaccine-derived poliovirus (VDPV) can occur and result in cases of paralytic polio.

Presently, there are a number of circulating VDPV outbreaks in Africa and other countries.

To combat this new challenge, new forms of OPV that are more genetically stable have been developed and will start to be used this year, which is anticipated to avoid the emergence of VDPV outbreaks.