Polio

Poliomyelitis is the paralytic disease caused by poliovirus, a small non-enveloped enterovirus spread faecal-orally. Most infections are silent, and only about 1 in 200 reaches the spinal cord, but when it does it destroys the motor neurons of the anterior horn and leaves permanent, asymmetric flaccid paralysis. That combination, a common infection with a rare but devastating outcome, has made polio one of the defining problems of twentieth-century public health.

Two organising ideas run through the topic. The first is the virus: three serotypes, a positive-sense RNA genome, entry through the gut, and a distinctive tropism for lower motor neurons. The second is the system built to defeat it: polio is the second disease, after smallpox, targeted for global eradication, controlled by mass vaccination and tracked by surveillance for acute flaccid paralysis (AFP). The endgame is unusually subtle because the live oral vaccine can itself revert to a paralysing, transmissible vaccine-derived poliovirus, so the closing stages turn as much on the vaccine as on the wild virus.

For expanded detail

→ See the Poliovirus profile for the virus itself: classification and structure, the replication cycle, pathogenesis, the clinical spectrum and the approach to a child with acute flaccid paralysis, diagnosis, and the oral and inactivated vaccines.

→ See Eradication and Surveillance for the programme and the system: elimination versus eradication, the Global Polio Eradication Initiative, certification, AFP and environmental surveillance, the vaccine-derived poliovirus problem, the vaccine switch, and containment.

The virus at a glance

Feature	Poliovirus
Family / genus	Picornaviridae / Enterovirus (species Enterovirus coxsackiepol)
Serotypes	Three (1, 2, 3); no cross-protection
Genome	Positive-sense single-stranded RNA, ~7.4 kb, one polyprotein
Receptor	CD155 (the poliovirus receptor)
Transmission	Faecal-oral; humans the only reservoir
Incubation to paralysis	~7 to 21 days (range 3 to 35)
Target tissue	Anterior-horn motor neurons of the spinal cord
Diagnosis	Virus detection from two stool specimens

The clinical spectrum

Poliovirus infection ranges from silent carriage to permanent paralysis, and the proportions matter as much as the syndromes.

Outcome	Approximate frequency	Features
Asymptomatic infection	Over 90%	No symptoms; virus shed in stool for weeks
Abortive poliomyelitis	~4 to 8%	Brief non-specific febrile illness
Non-paralytic poliomyelitis	Small minority	Aseptic meningitis, no weakness
Paralytic poliomyelitis	~1 in 200	Asymmetric flaccid paralysis; sensation spared
Post-polio syndrome	Up to a quarter of survivors	New progressive weakness decades later

Because wild poliovirus is now rare, most children presenting with acute flaccid paralysis have another cause, and each must still be investigated as possible polio. The features that point towards poliomyelitis are asymmetric, proximal, purely motor flaccid weakness with preserved sensation, rapid onset and fever, which help separate it from Guillain-Barre syndrome (symmetric, sensory, usually afebrile), transverse myelitis (a sensory level) and traumatic neuritis (one injected limb).

The two vaccines

Two vaccines control polio, and the difference between them shapes both practice and the eradication endgame.

Feature	OPV (oral, live attenuated)	IPV (inactivated, injected)
Gut (mucosal) immunity	Strong; blocks transmission	Weak
Systemic immunity	Good	Good
Interrupts community spread	Yes	Limited
Administration	Oral, no needle, low cost	Injection, trained staff
Can cause paralysis	Rarely (VAPP; and can revert)	No
Use in immunocompromise	Contraindicated	Safe

OPV is the oral poliovirus vaccine and IPV the inactivated poliovirus vaccine. OPV’s live nature carries two rare but defining risks: vaccine-associated paralytic poliomyelitis (VAPP), paralysis caused directly by the vaccine virus, and reversion to a vaccine-derived poliovirus (VDPV) that can circulate and paralyse like wild virus.

Vaccine-derived poliovirus

A Sabin vaccine strain is attenuated by only a few mutations, chiefly in domain V of the 5’ untranslated region, and these can revert in the gut and, on spread through undervaccinated communities, restore virulence. Vaccine-derived polioviruses are grouped by their setting:

Category	Setting
Circulating (cVDPV)	Community transmission shown; causes outbreaks; mostly type 2
Immunodeficiency-associated (iVDPV)	Chronic excreter with a B-cell immunodeficiency
Ambiguous (aVDPV)	No transmission or source shown, or an environmental isolate

Circulating type 2 vaccine-derived poliovirus now causes most of the world’s remaining paralytic polio, which is why the type 2 oral vaccine was withdrawn in the 2016 “switch” from trivalent to bivalent oral vaccine, IPV was added to routine schedules, and a genetically stabilised novel oral vaccine (nOPV2) was developed for outbreak response.

Eradication and surveillance at a glance

The Global Polio Eradication Initiative (GPEI), launched by the 1988 World Health Assembly, has cut cases by about 99.9%. Wild poliovirus type 1 remains endemic only in Afghanistan and Pakistan; wild types 2 and 3 have been declared eradicated, and the WHO African Region was certified free of wild poliovirus in 2020. Progress is confirmed by certification (national committees to the regional commission to the global commission) and sustained by surveillance against defined targets:

Indicator	Target
Non-polio AFP rate (children under 15)	At least 1 per 100,000 (2 in priority regions)
Stool adequacy	At least 80% of cases
Two stool specimens	24 to 48 hours apart, within 14 days of onset
Reverse cold chain	4 to 8 degrees Celsius, lab within 3 days

Every isolate undergoes intratypic differentiation to separate wild, vaccine and vaccine-derived virus, and remaining poliovirus stocks are held only in certified facilities under the Global Action Plan for containment (GAPIII/GAPIV).

Key terms

The abbreviations that recur across the topic.

Term	Definition
AFP (acute flaccid paralysis)	The syndrome of sudden floppy weakness that triggers a polio investigation in any child under 15.
OPV / IPV	Oral (live attenuated) and inactivated (injected) poliovirus vaccines.
VAPP	Vaccine-associated paralytic poliomyelitis, paralysis caused directly by oral vaccine virus.
VDPV / cVDPV	Vaccine-derived poliovirus, and its circulating (outbreak) form.
Intratypic differentiation	The laboratory step that distinguishes wild, Sabin and vaccine-derived poliovirus.
GPEI	The Global Polio Eradication Initiative, the WHO-led partnership driving eradication.
GAPIII / GAPIV	The WHO Global Action Plan governing poliovirus containment.

References and recommended reading

Global Polio Eradication Initiative. polioeradication.org. The programme source for eradication strategy and current poliovirus status.

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.

Romero JR. Enteroviruses. In: Richman DD, Whitley RJ, Hayden FG, editors. Clinical Virology. 4th ed. Washington (DC): ASM Press; 2016.