Eradication and Surveillance — Questions

• cVDPV. A circulating vaccine-derived poliovirus is oral-vaccine (Sabin) virus that has regained neurovirulence and transmissibility through mutation and then spread person to person in an undervaccinated population, causing outbreaks indistinguishable from wild polio. Most are type 2.
• Counter-vaccine. Novel oral poliovirus vaccine type 2 (nOPV2) is a Sabin type 2 vaccine engineered for genetic stability (its principal attenuating site is locked and reinforced), so it gives the mucosal, transmission-blocking immunity of an oral vaccine while reverting far less readily, and it is used for type 2 outbreak response.

• Family. Poliovirus is a member of the family Picornaviridae (genus Enterovirus).
• Endemic countries. Wild poliovirus remains endemic only in Afghanistan and Pakistan.
• Circulating wild types. Only wild type 1 still circulates; wild types 2 and 3 have been declared eradicated (in 2015 and 2019 respectively).

Global polio eradication is close but not complete, and the picture now has two distinct strands, wild virus and vaccine-derived virus.

• Wild poliovirus. Wild poliovirus type 1 remains endemic in only two countries, Afghanistan and Pakistan. Wild type 2 was declared eradicated in 2015 and wild type 3 in 2019, so type 1 is the only wild type still circulating.
• Vaccine-derived poliovirus. Circulating vaccine-derived poliovirus, overwhelmingly type 2, now causes most of the world’s remaining paralytic polio, chiefly in undervaccinated parts of Africa and the Eastern Mediterranean.
• Overall progress. Cases have fallen by about 99.9% since the Global Polio Eradication Initiative began in 1988.
• Certification. The WHO African Region was certified free of wild poliovirus in 2020; surveillance and immunisation continue everywhere because importation and vaccine-derived virus remain threats.

Satisfactory AFP surveillance means the system is both sensitive enough to detect every case that could be polio and reliable enough to investigate each properly.

• Specimen collection. Collect two stool specimens, 24 to 48 hours apart, both within 14 days of the onset of paralysis (the window of peak shedding). Keep them in a reverse cold chain at 4 to 8 degrees Celsius and deliver them to a WHO-accredited laboratory within 3 days. A case meeting these conditions is “adequately investigated”.
• Sensitivity. The non-polio AFP rate (target at least 1 per 100,000 children under 15, or at least 2 in priority regions) confirms the system is finding the expected background of non-polio paralysis and would therefore have caught polio.
• Specimen quality. At least 80% of cases must have adequate stools, with supporting laboratory indicators such as non-polio enterovirus isolation in at least 10% of specimens (a proxy that the cold chain held).
• Timeliness and closure. Cases are reported and investigated promptly, with a 60-day follow-up for residual paralysis and final expert classification.

The endgame is the plan to move from near-elimination to a permanently polio-free world, and its central problem is that the live oral vaccine can itself cause and sustain disease.

• Interrupt wild transmission. Finish interrupting wild poliovirus type 1 in Afghanistan and Pakistan through high routine coverage, supplementary campaigns and surveillance.
• Withdraw the oral vaccine in steps. Because oral vaccine can revert, it is removed serotype by serotype once each wild type is eradicated. The 2016 switch from trivalent to bivalent oral vaccine withdrew the type 2 component globally.
• Fill the gap with inactivated vaccine. Inactivated poliovirus vaccine was introduced into routine schedules to maintain type 2 immunity without live type 2 virus.
• Use safer outbreak tools. Genetically stabilised novel oral poliovirus vaccine type 2 (nOPV2) responds to type 2 outbreaks with far less risk of seeding new vaccine-derived virus.
• Contain and certify. Remaining poliovirus stocks are contained under the Global Action Plan, and eradication is confirmed through formal certification backed by sustained surveillance.

A complete answer explains why type 2 is a special case in the endgame, the immunity gap that OPV2 withdrawal created, and the tools now used to respond to type 2 outbreaks.

The type 2 problem

Wild poliovirus type 2 was declared eradicated in 2015, so keeping the type 2 component in oral vaccine meant accepting type 2 vaccine-associated paralysis and vaccine-derived virus for no benefit. In the 2016 “switch”, trivalent oral vaccine was replaced globally by bivalent (types 1 and 3), and inactivated poliovirus vaccine was added to routine schedules to preserve type 2 immunity without live type 2 virus.

The immunity gap and cVDPV2

Withdrawing live type 2 virus let population mucosal immunity to type 2 fall, while circulating vaccine-derived poliovirus type 2 (cVDPV2) seeded before and after the switch continued to spread, and it is now the dominant cause of paralytic polio worldwide.

Outbreak response

Outbreaks are found by AFP and environmental surveillance and answered with rapid, high-coverage campaigns. Response historically used monovalent type 2 oral vaccine (mOPV2), but this reintroduces live type 2 virus and can itself seed further cVDPV2. The current tool is novel oral poliovirus vaccine type 2 (nOPV2), engineered for genetic stability so it reverts far less readily, alongside containment of type 2 materials in secured facilities.

A complete answer covers what fractional dosing achieves, its immunological limits, and where it fits in outbreak response.

The case for fractional dosing. Giving a fifth of a full inactivated vaccine dose intradermally stretches a constrained global supply of inactivated vaccine across far more children, and studies show seroconversion and priming comparable to the full intramuscular dose. In a supply-limited emergency this lets a programme protect more people from paralysis with the vaccine available.

The immunological limit. Inactivated vaccine, at any dose, raises serum antibody that prevents paralytic disease but induces little intestinal mucosal immunity, so a vaccinated child can still be infected and shed virus. Fractional dosing does not change this: it protects individuals but does little to interrupt faecal-oral transmission.

Implications for vaccine-derived outbreaks. Because circulating vaccine-derived poliovirus spreads through the gut, controlling an outbreak needs the transmission-blocking mucosal immunity that oral vaccine provides. Fractional inactivated vaccine is therefore best understood as a dose-sparing way to protect against paralysis and extend coverage, complementing rather than replacing the oral vaccine used to stop transmission, and it remains constrained by the need for intradermal delivery and a cold chain.

Certification requires three consecutive years with no wild poliovirus in every country, under certification-standard surveillance, alongside high routine immunisation and readiness to respond to importation.

Shorter intervals would risk certifying a region while undetected transmission continued; the three-year rule allows for the low ratio of paralytic cases to infections.

An immunodeficiency-associated VDPV (iVDPV) arises in a person with a B-cell immunodeficiency who cannot clear the vaccine virus and excretes it for months to years, forming a hidden long-term reservoir.

A circulating VDPV shows community person-to-person transmission; an ambiguous VDPV has neither demonstrated transmission nor an identified immunodeficient source, or is an environmental isolate.

A VP1 divergence of about 1% (roughly 10 nucleotides) for types 1 and 3, or 0.6% (roughly 6 nucleotides) for type 2, defines a vaccine-derived poliovirus. That degree of drift implies prolonged replication and transmission well beyond a single vaccinee.

A single change or any detectable change would capture normal early vaccine evolution, while 50% divergence is far beyond even wild-type differences.

Eradication is the permanent reduction to zero of the worldwide incidence of infection, after which control measures can be stopped altogether. This is the goal for polio, as it was for smallpox.

Reduction to a locally acceptable level is control, and regional zero with continued control is elimination. The absence of the agent even from laboratories is extinction, a further state beyond eradication.

Two specimens are collected 24 to 48 hours apart, both within 14 days of the onset of paralysis, the window of peak viral shedding. Two separated specimens improve the chance of isolating an intermittently shed virus.

A single specimen, or specimens taken outside the 14-day window or after the 60-day review, does not meet the adequacy standard.

A predictable background of acute flaccid paralysis from other causes occurs in any child population, so a system that is finding that background (at least 1 per 100,000 children under 15, or 2 in priority regions) is sensitive enough to have caught polio. The rate is therefore the measure of surveillance sensitivity.

Specimen quality is measured separately by the stool adequacy rate, and laboratory indicators track turnaround and cold-chain integrity.

Intratypic differentiation, a real-time PCR followed where needed by VP1 sequencing, assigns the serotype and separates wild poliovirus, Sabin-like vaccine virus and vaccine-derived poliovirus. This distinction drives the entire public-health response.

Culture precedes differentiation, and specimen adequacy and cold chain are assessed at collection and receipt, not by this step.

nOPV2 is a Sabin type 2 vaccine engineered for genetic stability, with the domain V determinant locked and other changes that make reversion far less likely, so it gives oral-vaccine mucosal immunity while greatly reducing the risk of seeding new cVDPV2. It is used for type 2 outbreak response.

It remains an oral type 2 vaccine, not an injectable, a type 1 product, or a replacement for routine inactivated vaccine.

In April 2016 the world synchronously replaced trivalent oral vaccine with bivalent (types 1 and 3), withdrawing the live type 2 component after wild type 2 was eradicated, and introduced inactivated vaccine to maintain type 2 immunity.

Only three poliovirus serotypes exist, and vaccination continued; the change was to the oral vaccine’s composition, not its route.

At 60 days the case is re-examined to document whether flaccid weakness persists, which helps classify the case, and it matters most where the stool specimens were inadequate and the virological result cannot be relied on.

Specimens are collected far earlier (within 14 days), and notification and classification are separate steps in the investigation.

Certification runs upward: each National Certification Committee submits its evidence to the Regional Certification Commission, which certifies the region. In Africa this is the African Regional Certification Commission.

The Global Certification Commission certifies the world (and declared wild types 2 and 3 eradicated). The National Polio Expert Committee classifies AFP cases, and the National Authority for Containment oversees laboratory containment.

The 1988 World Health Assembly resolution to eradicate poliomyelitis launched the Global Polio Eradication Initiative. Global incidence has since fallen by about 99.9%.

The initiative is led by national governments; Rotary, UNICEF and the certification bodies are partners or mechanisms within it, not the source of the founding resolution.

L20B is a mouse cell line engineered to express the human poliovirus receptor (CD155), so it favours poliovirus and suppresses other enteroviruses, giving a clean isolate for typing and sequencing.

The broadly sensitive RD line is used alongside it to catch non-polio enteroviruses and confirm the specimen carried viable virus; neither line performs serology or sequencing.

Each Sabin strain carries a principal attenuating mutation in domain V of the 5’ untranslated region (nucleotide 480 in type 1, 481 in type 2, 472 in type 3), a structured part of the internal ribosome entry site. These positions revert rapidly in the gut, restoring the RNA structure and neurovirulence, with capsid mutations assisting.

The polymerase, VP4, the poly(A) tail and the protease sites are not the shared primary attenuating determinant.

Specimens are kept at 4 to 8 degrees Celsius and must not be frozen, reaching the laboratory within 3 days of collection. It is the “reverse” cold chain because samples travel cold from patient to laboratory, the opposite direction to vaccine.

Freezing and repeated freeze-thaw damage the virus, and warm transport lets it degrade; both reduce isolation and would falsely lower the detected poliovirus burden.

The African Region was certified free of wild poliovirus in 2020, after Nigeria, the last endemic country on the continent, completed three years with no wild virus.

The 2015 and 2019 dates belong to the global declarations of eradication of wild types 2 and 3 respectively, not the African regional certification.

After eradication the virus persists in laboratory and manufacturing stocks, so the number of facilities holding it is minimised and only certified poliovirus-essential facilities may retain it, each overseen by a National Authority for Containment.

General clinical laboratories, pharmacies and health centres must destroy or transfer any poliovirus materials rather than retain them, and type 2 oral-vaccine stocks were destroyed after the switch.

The six core partners are the WHO, Rotary International, the US CDC, UNICEF, the Gates Foundation and Gavi, led by national governments. Medecins Sans Frontieres is not among them.

Rotary drives advocacy and fundraising, UNICEF procures and delivers vaccine, and the Gates Foundation is a major funder.