Public health
Eradication and Surveillance
Last reviewed 7 July 2026
Poliomyelitis is the second human disease, after smallpox, that the world has resolved to eradicate. Eradication is possible here for reasons peculiar to the virus: humans are its only reservoir, most immunity can be delivered by simple oral or injected vaccines, and a distinctive paralytic syndrome gives a signal to track. The effort rests on two systems that must work together. The first is vaccination, driven to a coverage high enough to interrupt transmission. The second is surveillance, the search for the virus sensitive enough to prove, country by country, that it is truly gone. What makes polio eradication unusually subtle is that the live oral vaccine, the tool that did most of the work, can itself regain the ability to paralyse and to spread, so the closing stages are dominated as much by vaccine-derived virus as by the wild agent, and by the task of safely containing the poliovirus that remains in laboratories and vaccine plants.
Elimination versus eradication
The words are often used loosely, but a programme needs them precisely. Control is the reduction of disease to a locally acceptable level through continuing intervention. Elimination is the reduction to zero of cases (or of infection) in a defined geographical area, with control measures still required to prevent re-establishment, for example from importation. Eradication is the permanent reduction to zero of the worldwide incidence of infection, after which intervention measures can be stopped altogether. A further state, extinction, means the agent no longer exists even in a laboratory.
The distinction is not academic. A country that has eliminated polio still vaccinates and still watches, because the virus persists elsewhere and can be imported. Only global eradication, formally certified, allows the interventions to end, and even then only after the remaining laboratory and manufacturing stocks are secured. Polio is a candidate for eradication precisely because it meets the biological preconditions that smallpox met: no animal reservoir, an effective vaccine, and no long-term human carrier state that would hide the virus indefinitely.
The Global Polio Eradication Initiative
In 1988 the World Health Assembly resolved to eradicate poliomyelitis, and the Global Polio Eradication Initiative (GPEI) was launched to do it. It is a public-private partnership led by national governments, with six core partners: the World Health Organization (WHO), which coordinates strategy and management; Rotary International, which drives advocacy and fundraising; the United States Centers for Disease Control and Prevention (CDC), which supplies scientific and laboratory expertise; the United Nations Children’s Fund (UNICEF), which procures and delivers vaccine; and, joining later, the Gates Foundation and Gavi, the Vaccine Alliance. Since 1988 the global incidence of polio has fallen by about 99.9%.
The strategy has four components. The first is high routine immunisation of infants to build baseline population immunity. The second is supplementary immunisation activities, the mass campaigns (national and subnational immunisation days) that vaccinate every child in an area over a few days, independently of routine services, to raise immunity faster than transmission can exploit gaps. The third is surveillance, treated below. The fourth is mopping-up, focused house-to-house campaigns in the last reservoirs where transmission persists. On this strategy, wild poliovirus type 1 now circulates endemically in only two countries, Afghanistan and Pakistan; wild type 2 was declared eradicated in 2015 and wild type 3 in 2019.
Certification and the African region
Eradication is not self-declared; it is certified through a tiered structure. Each country convenes a National Certification Committee (NCC) that compiles the evidence and submits it upward. A Regional Certification Commission, the African Regional Certification Commission (ARCC) in the WHO African Region, reviews each country and certifies the region. A Global Certification Commission certifies the world, and it is this body that declared wild poliovirus types 2 and 3 eradicated.
The bar for certification is deliberately high. A region is certified only when every country has recorded no wild poliovirus for at least three consecutive years in the presence of certification-standard surveillance, alongside high routine immunisation to maintain population immunity, demonstrated readiness to respond to an importation, and a functioning NCC. On this basis the WHO African Region was certified free of wild poliovirus in August 2020, after Nigeria, the last endemic country on the continent, completed three years with no wild virus. Certification of wild virus is not the end: vaccine-derived poliovirus continues to circulate in parts of Africa, and surveillance and immunisation must be sustained until global eradication is complete.
Surveillance for acute flaccid paralysis
The programme cannot declare the virus gone unless it can reliably find it, and the core method is surveillance for acute flaccid paralysis (AFP). The logic is syndromic: rather than rely on a clinical diagnosis of polio, which is unreliable and would miss the silent majority, the system detects every child with the syndrome that polio can cause, sudden floppy paralysis, and investigates each one virologically. An AFP case is any child under 15 years of age with acute flaccid paralysis, or a person of any age in whom poliomyelitis is suspected. Every such case is reported and investigated, whatever the eventual diagnosis.
The system is monitored by two headline indicators that together show it is both sensitive and reliable. The non-polio AFP rate measures sensitivity: because a predictable background of acute flaccid paralysis from other causes (Guillain-Barre syndrome, transverse myelitis and others) occurs in any child population, a system that is finding that background is sensitive enough that it would have caught polio had it been present. The stool adequacy rate measures whether cases are being properly investigated. The core targets are:
| Indicator | Target |
|---|---|
| Non-polio AFP rate (children under 15) | At least 1 per 100,000 globally; at least 2 per 100,000 in priority regions |
| Stool adequacy | At least 80% of cases adequately investigated |
| Reporting timeliness | At least 80% of cases reported within 7 days of onset |
| Specimen arrival | At least 80% arriving at the reference laboratory within 3 days, under reverse cold chain |
| Final classification | At least 80% classified within 90 days of onset |
Stool adequacy is the specimen standard on which the whole system depends. Two stool specimens are collected from every AFP case, 24 to 48 hours apart, and both within 14 days of the onset of paralysis, the window of peak viral shedding. The specimens must be kept in a reverse cold chain at 4 to 8 degrees Celsius (samples travel cold from the patient to the laboratory, the reverse of the vaccine cold chain) and reach a WHO-accredited laboratory within 3 days of collection. A case is adequately investigated only when these conditions are met.
Two further steps close each investigation. A 60-day follow-up examination looks for residual paralysis, which is most important where the stool specimens were inadequate and the virological result cannot be relied on; where adequate stools cannot be obtained, specimens are taken from close contacts. Finally, a national expert committee assigns each case a final classification (confirmed polio, polio-compatible, discarded, or not a true AFP case). AFP surveillance is increasingly reinforced by environmental surveillance, discussed with the laboratory network below.
The polio laboratory network
Every specimen is processed within the Global Polio Laboratory Network (GPLN), a WHO-accredited, standardised, tiered system of national, regional reference and global specialised laboratories that use identical protocols and reagents so that a result means the same thing anywhere in the world. The laboratory’s job is not only to find poliovirus but to say which poliovirus it is, because the distinction between wild, vaccine and vaccine-derived virus drives the entire public health response.
Isolation uses a two-cell-culture system that trades sensitivity against specificity:
| Cell line | Nature | Role |
|---|---|---|
| L20B | Mouse cells engineered to express the human poliovirus receptor (CD155) | Selective for poliovirus; suppresses other enteroviruses, giving a clean isolate for typing and sequencing |
| RD | Human rhabdomyosarcoma cells | Broadly sensitive to enteroviruses; detects non-polio enteroviruses and confirms specimen viability |
A processed stool suspension is inoculated onto both lines and watched for cytopathic effect. Growth on L20B flags a probable poliovirus, which is then characterised by intratypic differentiation (ITD), a real-time PCR that assigns the serotype and separates Sabin-like (vaccine) virus from non-Sabin-like virus. Any non-Sabin-like isolate proceeds to VP1 gene sequencing, which measures how far it has drifted from the parent vaccine strain: an isolate whose VP1 has diverged by about 1% (roughly 10 nucleotides) for types 1 and 3, or 0.6% (roughly 6 nucleotides) for type 2, is classified as a vaccine-derived poliovirus. The network also carries its own quality indicators, notably that at least 10% of stool specimens should yield non-polio enteroviruses, a proxy showing the reverse cold chain kept the specimens viable, and that isolates are sent for ITD within 7 days and results reported within 28 days.
Because most poliovirus infection is silent, the programme increasingly supplements case-based surveillance with environmental surveillance, the systematic testing of sewage for poliovirus. A single wastewater sample integrates shedding from a whole catchment, so environmental surveillance can reveal circulation before any child is paralysed, and in settings where clinical surveillance is weak it can be many times more sensitive than AFP surveillance alone. It is now a standard early-warning layer, particularly for detecting imported wild or vaccine-derived virus.
Vaccine-derived poliovirus
The defining difficulty of the endgame is that the oral poliovirus vaccine (OPV), live and attenuated, can regain virulence. OPV is attenuated by a small number of mutations, and because it replicates in the gut it can mutate and, on prolonged transmission through undervaccinated populations, evolve back towards the neurovirulence and transmissibility of wild virus.
Two distinct harms follow. Vaccine-associated paralytic poliomyelitis (VAPP) is paralysis caused directly by the vaccine virus in a recipient or a close contact, an individual, non-transmissible event occurring at a rate on the order of one per several hundred thousand to one per few million doses. Vaccine-derived poliovirus (VDPV) is the population-level problem: reverting virus that has regained the capacity to circulate and paralyse, behaving like wild poliovirus.
The molecular basis of reversion is well mapped. A principal attenuating mutation of each Sabin strain sits in domain V of the internal ribosome entry site, a structured region of the 5’ untranslated region, at nucleotide 480 in type 1, 481 in type 2 and 472 in type 3, supported by attenuating changes in the capsid. These positions revert rapidly in the gut of vaccinees, restoring the RNA structure and neurovirulence, and further virulence is recovered through capsid mutations and through recombination with other species C enteroviruses in the gut. Sabin type 2 is the least stable of the three and the main source of circulating vaccine-derived virus, while type 3 also reverts readily through its two-mutation attenuation; type 1, attenuated by many more mutations, is the most stable.
VDPVs are classified by their epidemiological setting:
| Category | Setting | Public health meaning |
|---|---|---|
| Circulating VDPV (cVDPV) | Community transmission demonstrated | Behaves like wild virus; causes outbreaks; the dominant remaining cause of paralytic polio |
| Immunodeficiency-associated VDPV (iVDPV) | A person with a B-cell immunodeficiency who excretes virus for months to years | A hidden long-term reservoir; a risk to a polio-free world |
| Ambiguous VDPV (aVDPV) | No transmission shown and no immunodeficient source identified, or an isolate from the environment | Requires investigation and monitoring |
Circulating VDPVs emerge where OPV coverage is low, are overwhelmingly type 2, and now cause most of the world’s paralytic polio, which is why the vaccine strategy itself had to change.
The end of the oral vaccine: the switch and new tools
Once wild type 2 was eradicated, continuing to use the type 2 component of the oral vaccine meant accepting a growing burden of type 2 VAPP and cVDPV2 for no benefit against a virus that no longer existed in the wild. In April 2016 the world therefore carried out the “switch”, a globally synchronised change from trivalent OPV (types 1, 2 and 3) to bivalent OPV (types 1 and 3), withdrawing the live type 2 component everywhere at once, the largest coordinated vaccine change ever attempted. To preserve immunity against type 2 without live type 2 virus, inactivated poliovirus vaccine (IPV) was introduced into routine schedules worldwide.
The switch created its own difficulty. Population immunity to type 2 fell as the live vaccine was withdrawn, while cVDPV2 seeded before and after the switch continued to circulate, and outbreak response with monovalent type 2 OPV sometimes seeded further cVDPV2, a self-perpetuating cycle. The response was a new tool: novel oral poliovirus vaccine type 2 (nOPV2), a type 2 vaccine engineered for genetic stability, with the domain V determinant locked and other modifications that make reversion far less likely. Authorised for emergency use from 2020 and now given in very large numbers, nOPV2 provides the mucosal, transmission-blocking immunity of an oral vaccine while greatly reducing the risk of seeding new outbreaks. The long-term logic of the endgame is to stop all use of oral vaccine once transmission is interrupted, relying on IPV, because as long as any OPV is used the vaccine-derived problem cannot fully end.
Poliovirus containment
Eradicating the virus in nature does not remove it from the world: poliovirus persists in laboratory freezers, vaccine-manufacturing plants, and archived clinical and environmental specimens. After each type is eradicated, this stored virus becomes the main route by which it could be reintroduced, whether by laboratory accident, an unrecognised infection in a worker, or improper waste disposal. Containment is the systematic reduction and safeguarding of these stocks.
The framework is the WHO Global Action Plan for poliovirus containment (GAPIII, and its successor GAPIV), which sets the biorisk-management standards for any facility retaining poliovirus materials. The number of such facilities is minimised, and those that remain are designated poliovirus-essential facilities (PEFs), which must meet strict safeguards and be formally certified under a containment certification scheme. Under a 2018 World Health Assembly resolution, each country retaining poliovirus appoints a National Authority for Containment (NAC) to oversee its PEFs. In practice containment reaches across several settings: community health facilities and diagnostic laboratories that may unknowingly hold poliovirus in stored stool or respiratory specimens; environmental samples and sewage archives; pharmacy and programme stocks, above all the destruction of remaining type 2 oral vaccine after the switch; and laboratories, where clinical-pathology and virology laboratories must either destroy poliovirus materials or transfer them to a certified PEF. The aim is simple: as immunity is deliberately allowed to wane in a polio-free world, the consequence of any escape rises, so the residual virus must be held only where it can be securely managed.
South African context
South Africa has had no indigenous wild poliovirus for decades (the last wild poliovirus was detected in 1989) and was certified free of wild poliovirus as part of the WHO African Region in 2020. The standing risk is importation, of wild virus from the remaining endemic countries or, more immediately for the continent, of circulating vaccine-derived poliovirus from outbreaks elsewhere in Africa. This is why the country maintains high vaccination coverage and sensitive surveillance rather than treating polio as a closed chapter.
The eradication obligations are overseen by a set of national bodies. The National Certification Committee (NCC) compiles and endorses the evidence that South Africa remains polio-free and submits it to the ARCC. The National Polio Expert Committee (NPEC) reviews and assigns the final classification of every AFP case. The National Task Force on poliovirus containment (NTF) and the National Authority for Containment (NAC) implement GAPIII/GAPIV containment and oversee any poliovirus-essential facility. Laboratory work is centralised at the National Institute for Communicable Diseases (NICD), the only WHO-accredited poliovirus laboratory in the country, which serves South Africa and several neighbouring states and runs the L20B and RD culture system, intratypic differentiation and sequencing. South Africa also runs environmental (wastewater) surveillance, piloted from 2018 in Tshwane and Johannesburg and now extended to more than 26 sampling sites in the major metropolitan areas and higher-risk districts.
The South African AFP specimen pathway
Because the NICD is the sole accredited laboratory, all specimens from any case or suspected case of AFP must be sent to the NICD. The protocol is exact:
- Collect two stool specimens, 24 to 48 hours apart, both within 14 days of the onset of paralysis. Where stool cannot be obtained, submit a rectal swab.
- Maintain the reverse cold chain throughout. Place specimens immediately into a dedicated vaccine carrier or cool box with frozen ice-packs, or refrigerate; the first specimen may be refrigerated until the second is collected, and if no refrigeration is available the specimen must be delivered without delay. Keep at 4 to 8 degrees Celsius and do not freeze.
- Deliver to the NICD within 3 days of collection, transported in the vaccine carrier with frozen ice-packs. The NICD receiving office must be notified in advance so the consignment can be tracked, and referral NHLS laboratories should forward the cool box without opening it, to protect both containment and the cold chain.
- Complete the Case Investigation Form (CIF) in full to accompany the specimens. The provincial Department of Health assigns the epidemiological (“Epid”) number, and the form records the clinical features (site of paralysis, onset date, whether the paralysis was sudden, flaccid and asymmetric, fever at onset), the OPV vaccination history, the specimen details and the 60-day follow-up.
- Follow up at 60 days to document residual paralysis, and refer the completed case to the NPEC for final classification.
The national surveillance targets mirror the global ones, applied with a higher local sensitivity aim: a non-polio AFP rate target of the order of 2 per 100,000 children under 15 (with an internal programme aim above this), stool adequacy and NPEV isolation rates that have historically exceeded the WHO benchmarks, and zero poliovirus isolated.
References and recommended reading
Global Polio Eradication Initiative. polioeradication.org. The authoritative programme source for the eradication strategy, the partnership, current wild and vaccine-derived poliovirus status, the 2016 switch and nOPV2.
World Health Organization. Global guidance for conducting acute flaccid paralysis (AFP) surveillance in the context of poliovirus eradication. 2nd ed. Geneva: World Health Organization; 2026. The authoritative source for the AFP case definition, the surveillance-quality indicators and stool adequacy.
World Health Organization Regional Office for Africa. Polio certification. The source for the certification hierarchy and the 2020 certification of the African Region as free of wild poliovirus.
World Health Organization. Global Action Plan to minimize poliovirus facility-associated risk (GAPIV). 4th ed. Geneva: World Health Organization; 2022. The source for the poliovirus containment framework, poliovirus-essential facilities and the National Authority for Containment.
Coyne CB, Oberste MS, Pallansch MA. Enteroviruses: polioviruses, coxsackieviruses, echoviruses, and newer enteroviruses. In: Howley PM, Knipe DM, editors. Fields Virology. 7th ed. Philadelphia: Wolters Kluwer; 2023. The source for the molecular basis of Sabin attenuation and vaccine-derived poliovirus reversion.
Romero JR. Enteroviruses. In: Richman DD, Whitley RJ, Hayden FG, editors. Clinical Virology. 4th ed. Washington (DC): ASM Press; 2016. The source for the vaccine-derived poliovirus categories and the laboratory diagnosis of poliovirus.
National Institute for Communicable Diseases / National Department of Health, South Africa. Guidelines for the collection, handling and transport of faecal samples for AFP surveillance, and the AFP Case Investigation Form. The source for the South African AFP specimen pathway.
Manyanga D, Maseti E, Mokoena K, et al. Assessment of environmental surveillance for the detection of poliovirus implementation in the metropolitan districts of South Africa, 2020-2023. Pan African Medical Journal. 2025;51(58). The source for the South African environmental surveillance programme.