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It has been 35 years since the World Health Assembly passed a resolution to eradicate polio. In 1988, it was estimated that nearly 1,000 people were paralysed by wild poliovirus each day worldwide. Thankfully, tremendous progress has been made since then, with only nine cases of wild polio reported so far this year in the two remaining countries where the virus is still endemic: Afghanistan and Pakistan.

This achievement is significant, considering that there are three distinct strains of wild poliovirus, with the World Health Organization certifying the global eradication of types 2 and 3 in 2015 and 2019, respectively.

Vaccination is key to stopping the transmission of wild poliovirus transmission, by maintaining high rates of polio vaccination. Two polio vaccines are currently available: the inactivated Salk polio vaccine, which is administered in Australia, and the live, attenuated Sabin oral polio vaccine, that is used by many countries at risk of a polio outbreak.

The Sabin polio vaccine offers a significant advantage as it mimics a natural wild poliovirus infection by growing in the gut and triggering mucosal immunity in the lining of the gut. However, it comes with a drawback, it can also mimic the wild poliovirus by incorporating small changes in its genetic sequence as part of its normal replication cycle. In areas with low rates of polio vaccination, this can lead to vaccine-derived poliovirus (VDPV), a variant of the virus that can infect one unvaccinated individual after another until it reverts to a more virulent, non-weakened form.

The occurrence of VDPV outbreaks lead to the development of a genetically more stable form of the Sabin oral polio vaccine, called the novel oral polio vaccine or nOPV. So far, nOPV is in use for type 2 poliovirus, with ongoing development for nOPV strains designed for poliovirus types 1 and 3.

Clinical surveillance for cases of acute flaccid paralysis, a polio-like illness in children, was introduced in Australia in 1995. Australia’s Polio Expert Panel monitors the country’s polio-free status by regularly reviewing clinical and laboratory data generated from the Acute Flaccid Paralysis Surveillance Program. This assessment helps determine if a case is compatible with polio or some other disease. Nonetheless, a major challenge to achieving wild polio eradication is that most poliovirus infections do not manifest noticeable symptoms, so wild poliovirus could be circulating without causing paralysis.

Poliovirus infects the gut and is shed in faeces for weeks, making it detectable in sewage. This is one of the reasons why, in 2010, surveillance for poliovirus was expanded in Australia to include the testing of wastewater. While many Australians only became aware of the benefits of wastewater surveillance during the COVID-19 pandemic, the World Health Organization’s Global Polio Laboratory Network, which includes the National Enterovirus Reference Laboratory at the Doherty Institute, has been utilising this form of viral surveillance for many years.

In 2022, the importance of wastewater surveillance came into sharp focus as instances of VDPV type 2 were detected in wastewater in Canada, England, Israel and the United States, and later determined to be linked by genetic sequencing of viruses from each country.

With the last remaining reservoir of wild poliovirus type 1 contained to just two countries, the global eradication of poliovirus is in reach. The challenge lies in maintaining high levels of polio vaccination worldwide while limiting further VDPV outbreaks until the eradication of wild poliovirus is certified.