It has been 25 years since the World Health Organization (WHO) declared the Western Pacific Region, including Australia, to be polio-free in 2000.1 While the region has maintained its wild poliovirus-free status each year since then, as assessed by the WHO Polio Regional Certification Commission, recent global events concerning vaccine-derived poliovirus (VDPV) outbreaks and results of wastewater testing are a reminder that the region cannot be complacent about the risk of polio importations and outbreaks until the global certification of polio eradication. Australia has well-established national surveillance systems for poliovirus types 1, 2 and 3, supported by the Australian Government Department of Health, Disability and Ageing. This includes syndromic surveillance for cases of acute flaccid paralysis (AFP) in children younger than 15 years of age, wastewater surveillance and enterovirus surveillance and a poliovirus detection outbreak response plan.2, 3 AFP represents the tip of the iceberg in relation to poliovirus infections, with less than 1% causing paralysis and approximately 90% remaining asymptomatic. For this reason, it is critical all AFP cases are fully investigated, which involves notification of cases, completion of an online clinical questionnaire and the collection of adequate stool specimens from the patient (Fig. 1). Australia has two clinical surveillance systems for AFP: one coordinated by the Australian Paediatric Surveillance Unit (APSU) and the other by the Paediatric Active Enhanced Disease Surveillance (PAEDS) network.4, 5 AFP surveillance was established by APSU in 1995, and more than 1300 paediatricians and clinicians are enrolled nationally to notify rare childhood diseases, complications of common diseases or adverse effects of treatment each month, and, in addition for AFP cases, complete an online clinical questionnaire and arrange for collection of adequate stool specimens; https://my.fuzee.com/apsu-vidrl/afpquestionnaire.html. PAEDS was established in late 2007 and is now based at eight tertiary paediatric hospitals in Adelaide (Women's and Children's Hospital), Brisbane (Queensland Children's Hospital), Darwin (Royal Darwin Hospital), Melbourne (Monash Medical Centre and The Royal Children's Hospital), Perth (Perth Children's Hospital) and Sydney (The Sydney Children's Hospital and The Children's Hospital Westmead). PAEDS nurses routinely review the hospital records for cases of AFP, collate the clinical history and arrange for collection of adequate stool specimens. While most AFP cases are ascertained by the PAEDS network, APSU clinicians play an important role in notifying AFP cases at regional hospitals not served by PAEDS. Duplicate notification of AFP cases by both the APSU and the PAEDS network is encouraged to increase the sensitivity of the national AFP surveillance programme, which is coordinated by the National Enterovirus Reference Laboratory at the Victorian Infectious Diseases Reference Laboratory.2 Syndromic surveillance, rather than the diagnosis of poliomyelitis, enables the sensitivity of the surveillance system to be measured in polio-free countries. The international AFP surveillance targets set by the WHO include (i) reporting at least one non-polio AFP case per 100 000 children younger than 15 years of age and (ii) laboratory testing of adequate stool specimens, which are defined as collection of two specimens more than 24 h apart, and within 14 days of the onset of paralysis, due to intermittent and declining amounts of virus shed over 6 weeks, from at least 80% of the non-polio AFP cases. Australia has exceeded the non-polio AFP rate for the past 18 years (2008–2025) but has never reached the rate of adequate stool specimen collection of 80% of AFP cases, averaging 64% from 2020 to 2024. The WHO regards Australia's low rate of stool collection to be a gap in the country's surveillance system as laboratory testing at the WHO-accredited National Enterovirus Reference Laboratory is used to confirm or exclude a causal association of AFP with poliovirus infection, in conjunction with the clinical data. Afghanistan and Pakistan are the last two countries where wild poliovirus type 1 is endemic and remain a risk as the source of polio importations, as occurred in Australia in 2007 and China in 2011.6, 7 In 2014, the WHO declared the international spread of poliovirus to be a Public Health Emergency of International Concern (PHEIC), under the International Health Regulations (IHR 2005), which has been renewed every 3 months since then.8 One outcome of the PHEIC declaration is an emphasis on the need for polio vaccination of all residents and long-term visitors residing in polio-endemic countries and countries with polio outbreaks, with inactivated polio vaccine (IPV) or oral polio vaccine (OPV) between 4 weeks and 12 months prior to international travel. Since the PHEIC declaration, the only importation of AFP cases due to wild poliovirus from the endemic countries was from Pakistan to Malawi in 2021, which spread to Mozambique in 2022, with a total of nine cases reported by the two countries. The foundation of global polio eradication is vaccination. Humans are the only natural host of poliovirus, and person-to-person transmission of wild poliovirus through faecal-oral transmission can be stopped by maintaining high levels of polio vaccination, which has already led to the certification of eradication of wild poliovirus types 2 and 3 in 2015 and 2019 respectively.9 All countries now include IPV in their immunisation schedule, with many such as Australia using IPV exclusively. However, the two polio-endemic countries and others at high risk of polio importations and outbreaks still use OPV containing live attenuated Sabin poliovirus vaccine strains mimicking natural infection of the gastrointestinal tract necessary to limit virus shedding. Wild and vaccine strains of poliovirus incorporate mutations in the genome as a natural feature of the replication cycle, due to the viral enzyme that produces copies of the genome, RNA-dependent RNA polymerase, lacking proofreading ability. OPV-using countries with poor water and sanitation infrastructure that do not maintain high levels of OPV coverage are at risk of polio outbreaks due to the emergence of circulating VDPV (cVDPV) through person-to-person transmission leading to an accumulation of mutations that result in loss of attenuation in OPV strains and reversion to neurovirulence.10 Paradoxically, most polio cases worldwide are now caused by type 2 VDPVs rather than wild polioviruses. For example, in 2024, there were 99 cases of wild poliovirus AFP compared to 463 cases of cVDPV AFP in 14 countries, with 448 cases caused by cVDPV2.11 The high number of polio outbreaks from multiple emergences of cVDPV2 from 2006 onwards led to the removal of poliovirus type 2 from trivalent OPV in 2016, known as the switch to bivalent OPV containing Sabin poliovirus types 1 and 3 in 155 countries using trivalent OPV.12 In preparation for the switch, in 2013, the WHO recommended all countries introduce at least one dose of IPV in the routine immunisation schedule to maintain immunity to poliovirus type 2, with a further recommendation in 2020 for at least two doses of IPV. It is now acknowledged that the switch to bivalent OPV was a failure that did not prevent outbreaks of cVDPV2 due to a number of factors including (i) insufficient supply of IPV for routine immunisation and outbreaks, (ii) gaps in pre-switch poliovirus type 2 immunity, (iii) undetected cVDPV2 transmission at the time of the switch, (iv) limited supply of monovalent OPV2 for response to cVDPV2 outbreaks and (v) late detection of cVDPV2 outbreaks delaying implementation of control measures.13 Subsequently, a novel oral polio vaccine type 2 (nOPV2) was developed as a more genetically stable version of the Sabin poliovirus vaccine strain and approved for use in 2020. The genetic modifications to produce nOPV2: (i) stabilised the main site of attenuation of Sabin poliovirus strains in the 5′ untranslated region; (ii) relocated a critical genetic element to reduce increased virulence due to genetic recombination with non-polio enteroviruses; and, (iii) mutated the nucleic acid sequence encoding the RNA-dependent RNA polymerase to increase the fidelity of genomic replication.14 Since its introduction in 2021, nOPV2 has become the preferred vaccine to use in response to cVDPV2 outbreaks and while it has also caused polio outbreaks due to emergence of VDPV (referred to as cVDPV2-n), modelling data estimate the risk of a VDPV2 outbreak from usage of nOPV2 is 82% less than with usage of the original Sabin poliovirus type 2 vaccine.15 In 2023, Indonesia reported a cVDPV2-n outbreak in the Papua provinces after using nOPV2 in response to a Sabin-derived cVDPV2 outbreak in other regions of the archipelago. The last cVDPV2-n detection in Indonesia was in June 2024, but in April 2025, cVDPV2-n related to the Indonesian outbreak was first isolated from wastewater in Papua New Guinea and subsequently associated with five AFP cases reported up to December 2025.11 Poliovirus is an ideal candidate for environmental or wastewater surveillance, given the asymptomatic nature of infection, shedding of virus in faeces and its relative stability in the environment, which can be used to monitor for poliovirus importations in polio-free countries and the emergence of VDPV and determine the extent of transmission during polio outbreaks. Members of the WHO Global Polio Laboratory Network were already performing environmental surveillance for poliovirus when the network was established in 1989, and, in 2025, 11 countries in the WHO Western Pacific Region were routinely testing environmental samples for poliovirus. Wastewater surveillance served to link transmission of cVDPV2 in Canada, England, Israel and the United States in 2022, and more recently identified repeated importations of a different emergence of cVDPV2 in England, Finland, Germany, Poland and Spain in 2024.16, 17 The National Enterovirus Reference Laboratory established environmental surveillance for poliovirus in Australia in 2010 and routinely isolates Sabin poliovirus types 1 and 3 associated with recent vaccination with OPV in returned travellers or visitors from OPV-using countries.2 In 2017, VDPV2 was isolated in wastewater from Melbourne and, based on the genetic sequence, was most likely shed from a person with chronic poliovirus infection due to a primary immunodeficiency, but was classified as ambiguous VDPV2 because the source could not be confirmed.18 People with primary immunodeficiencies involving a lack of B cells are at risk for chronic poliovirus infection if exposed to OPV through vaccination or close contact, potentially leading to immunodeficiency-related VDPV. The WHO recommends implementing clinical surveillance to detect chronic poliovirus infection in people with primary immunodeficiency, and the National Enterovirus Reference Laboratory is undertaking a pilot study in Melbourne.19 Since the programme started in 1988, the Global Polio Eradication Initiative has achieved major milestones by eradicating wild poliovirus types 2 and 3 worldwide and reducing the number of polio cases by more than 99.9% and the number of endemic countries from 125 to two. Nevertheless, there are complex challenges to address before the ambitious goal of global certification of wild poliovirus can be declared, followed by eradication of VDPVs and cessation of OPV usage. Robust surveillance systems are required to identify and fully characterise poliovirus in all its guises both from cases of AFP and environmental samples: wild poliovirus type 1, Sabin poliovirus types 1 and 3, nOPV2, cVDPV2-n and ambiguous, circulating and immunodeficiency-related VDPV types 1, 2 and 3.20 Polio anywhere is a threat to children everywhere. The National Poliovirus Surveillance Programme performed by the National Enterovirus Reference Laboratory and the Acute Flaccid Paralysis Surveillance Programmes performed by the Australian Paediatric Surveillance Unit and the Paediatric Active Enhanced Disease Surveillance Network are funded by the Australian Government Department of Health, Disability and Ageing. The data that support the findings of this study are available in the WHO's global polio database at https://polioeradication.org/. These data were derived from the following resources available in the public domain: (a) https://polioeradication.org/wild-poliovirus-count/, (b) https://polioeradication.org/circulating-vaccine-derived-poliovirus-count/.
Thorley et al. (Sun,) studied this question.