A few things about poliomyelitis (‘polio’) and poliovirus, the enterovirus that causes this disease.

What is Polio?

Polio is the name used to describe a paralytic disease. It is caused by infection with one of three human enteroviruses, poliovirus 1, 2, and 3 (PV1, PV2, and PV3). Polio is a severe disease, and the poliovirus is a disease-causing virus. Polio mostly affects children and is a human disease.[9] PV enters through the mouth, replicates in the throat and intestine and can spread system-wide, infecting the brain and spinal cord. PV can be shed from an infected person’s throat and gut for weeks.

Different outcomes after infection

Around three-quarters of people infected with PV have mild or no symptoms. About a quarter feel poorly for less than a week.[6]

A smaller group develop signs that their brain and spinal cord have become infected.[6,7] They feel pins and needles in their legs or experience meningitis (inflammation of the lining of the brain; in approximately 1 in 25, about 4%).

In about 1 in 200 people (0.5%), paralysis develops (weakness in arms, legs, or both). This is poliomyelitis; what we call polio. The name doesn’t actually apply to the less severe infection outcomes.

Two to 10 of every 100 people with paralysis (0.01% to 0.05% of all PV infections) will die.[6]

There’s also post-polio syndrome, involving muscle pain, weakness or paralysis that can develop 15 to 40 years later, even in those who, as children, seemed to recover at the time.

PV vaccines did wonders

That a virus caused polio has been known since 1908.[3] The first vaccines were developed in 1955, and in 1988, the Global Polio Eradication Initiative (GPEI) was launched. GPEI aimed to interrupt PV transmission through vaccination.

The Americas were declared free of endemic polio in 1994, and the Western Pacific in 2000. The South-East Asia region, which includes India, was declared polio-free in 2014.

The majority of those vaccinated are protected from disease.[6] Polio cases have dropped by 99.9% since 1998.[5]

Wild PV1 is still detected in Pakistan (as of 29APR2026). Wild PV2 was eradicated in September 2015 (last detected in 1999, India).[5] Wild PV3 hasn’t been detected anywhere since November 2012.[5]

Pretty clearly, #vaccineswork.

The polio vaccine

The easiest-to-use, most inexpensive, and needle-free oral polio vaccine (OPV) comes in different formulations.[10] It uses a weakened but live form of the virus. It is a PV that can replicate in the gut and enter the bloodstream, but it doesn’t cause spinal cord and brain disease. This builds strong immunity, preventing any future naturally acquired infection from developing into a disease.[8] In fact, the vaccinated person sheds vaccine PV for a time, thereby infecting others and helping immunise a community.

There is also the more expensive, injectable Inactivated Polio Vaccine (IPV). IPV creates antibodies through its presence in our bloodstream, but it doesn’t replicate in the gut like the OPV vaccines. This immunity is weaker. It protects the person from disease, but it doesn’t prevent them from shedding PV should they be subject to a naturally acquired infection.

We shed the virus after infection & OPV

After being vaccinated using OPV, we poop out the virus, as we would with a wild infection, for weeks.[8]

Naturally acquired infection is unlikely to occur in a community that has been immunised through vaccination. But in the last regions of the world with low vaccination rates, the risk of natural infection rises. Which brings us to the less welcome news.

Vaccine-derived PVs

Rarely – about 3 times per million births [10] – in regions that have lots of unvaccinated people, the process of infecting others through shedding OPV vaccine virus may go on too long. It passes from person to person to person to person. In each person, the PV may change slightly. Viruses that can replicate are always mutating. If that process of infection and mutation continues over 12-18 months, a mutated PV may emerge that can once again infect the nerves and brain; a neurovirulent circulating Vaccine-Derived PolioVirus (cVDPV).[8] In 2025-2026, cVDPV1 case was found in the Democratic Republic of the Congo, cVDPV2 cases found in Chad, the Democratic Republic of the Congo, Malawi and Nigeria and cVDPV3 cases in Chad and Nigeria.

Sometimes, communities with these low vaccination levels may also have poor access to water, so sanitation suffers, increasing the likelihood of exposure to the shed virus.

This is a reason why OPV vaccinations tend to happen on a massive scale in a large susceptible population. If everyone is vaccinated at a similar time, they will be protected from infection well before 12-18 months pass. But if there is resistance to vaccination or violence, pockets of unvaccinated people can remain. A cVDPV outbreak can be quickly stopped with vaccination.

So why use OPV at all?

Why not switch over to IPV completely? That’s because the OPV vaccine produces a higher level of immune protection. It prevents PV shedding, plus it’s a transmissible vaccine.[11,12] Use of OPV is important for the elimination of polio from a large environment. IPV is used together with OPV in some areas. In areas where polio transmission has already been stopped and the risk of natural infection is very, very low, IPV is used alone. Use of OPV in Australia stopped in November 2005.

It is nothing short of an astounding amount of work to ensure that so many people have been vaccinated in so many places worldwide and that we have nearly eradicated this virus from circulation.

Poliovirus is part of a big virus group

The PVs belong to one species of the genus Enterovirus, called Enterovirus coxsackiepol. There are 27 viruses or isolates in that species and 14 other species in the genus. Most enterovirus species contain viruses that don’t infect humans – as far as we know.

PVs are perhaps the most well-known of the enteroviruses associated with AFP, with or without long-term consequences. But other enteroviruses are also found in children with AFP.

The disease caused by another EV, EV-A71 (species A, number 71), has several licensed vaccines produced in China. The serious diseases associated with EV-A71 include hand-foot-and-mouth disease, neurological disorders, encephalitis, and AFP.

Other enteroviruses found in children with AFP include Coxsackie virus (CV) A24, CV-B5, enterovirus (EV) 75, echovirus (E) 9, E-11 and E-18.[15] These viruses sit across two different species, Enterovirus betacoxsackie and Enterovirus coxsackipol.

But what does it mean to have this AFP thing?

Acute flaccid paralysis = sudden onset, floppy, inability to work muscles

AFP is a clinical description of a combination of signs and symptoms that occur together (a syndrome).

AFP has many causes, for example, this description from 1970.[16]

It isn’t a new term. Here are descriptions of it in articles published in 1899-1900.[17,21]

AFP is the rapid onset of a floppy paralysis.[9] It’s a weakness in one or more limbs or the respiratory muscles, resulting from damaged lower motor neurons or cranial nerves.[13] If damage involves the respiratory muscles it can render the person unable to breathe without mechanical help. When long-lasting or permanent, muscle wastage occurs.

Paralysis isn’t always permanent

The acute phase of illness lasts up to a month. When recovery of paralysed muscles occurs, it does so within six months to two years.[24] After this, what paralysis remains is described as “post polio residual paralysis” and is lifelong.

Acute flaccid paralysis has other causes as well

There are other, unrelated viruses linked with AFP.
[18,19] These include:

• West Nile virus
• Japanese encephalitis virus
• Human herpesviruses
• European tick-borne encephalitis virus.

Apart from viruses, there are links between AFP and conditions including [16,20,24]:

• Guillain-Barré syndrome
• Transverse myelitis
• Traumatic neuritis
• Toxic neuropathies
• Muscle disorders
• Systemic diseases
• Demyelinating diseases
• Botulism
• Insecticide poisoning
• Snake bites

You may also have heard of acute flaccid myelitis (AFM) – this is a better-characterised subset of AFP, with more diagnostic imaging used to demonstrate spinal cord inflammation.[25] You’ve probably heard of AFM in the context of EV-D68 (species D, number 68), but AFM is also linked to EV-A71 and other viruses. AFM is a newer term, although what it describes is not.

Wrap up

Polio is only polio if poliovirus can be detected (and a lot of work goes into seeking it). The use of a carefully contemplated and constantly curated vaccine strategy, along with extensive collaboration and philanthropy, has nearly obliterated PV from the planet.

Another of my posts – But 40,000+ cases of AFP in India must mean polio is thriving, right? No. – looks at a different aspect of this. Why thousands of cases of AFP in India aren’t due to polio but are yet another benefit from the GPEI; enhanced surveillance for AFP.

References

1. Immunization Action Coalition
   http://www.immunize.org/photos/polio-photos.asp
2. Center for Disease Control and Prevention, (CDC) Public Health Image Library (PHIL), 3D poliovirus virion
   https://phil.cdc.gov/Details.aspx?pid=22498
3. Rotary | End Polio Now
   www.endpolio.org
4. Global Polio Eradication Initiative
   http://polioeradication.org/who-we-are/our-mission/
5. GLOBAL ERADICATION OF WILD POLIOVIRUS TYPE 2 DECLARED
   http://polioeradication.org/news-post/global-eradication-of-wild-poliovirus-type-2-declared/
6. What Is Polio? | CDC
   https://www.cdc.gov/polio/about/index.htm
7. What is polio? | Khan Academy
   https://www.khanacademy.org/science/health-and-medicine/infectious-diseases/polio/a/what-is-polio
8. VACCINE-DERIVED POLIOVIRUSES
   http://polioeradication.org/polio-today/polio-prevention/the-virus/vaccine-derived-polio-viruses/
9. COFFEE WITH POLIO EXPERTS: CAROLYN SEIN, WHO
   http://polioeradication.org/news-post/coffee-with-polio-experts-carolyn-sein-who/
10. Oral poliovirus vaccine | GPEI
    http://polioeradication.org/polio-today/polio-prevention/the-vaccines/opv/
11. Controlling epidemics with transmissible vaccines
    https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0196978
12. Transmissible Viral Vaccines
    https://www.cell.com/trends/microbiology/fulltext/S0966-842X(17)30212-3?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0966842X17302123%3Fshowall%3Dtrue
13. Polio will go, acute flaccid paralysis will stay
    https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(14)61080-1/fulltext
14. Acute Flaccid Paralysis (AFP) surveillance: the surveillance strategy for poliomyelitis eradication | WHO India
    http://www.searo.who.int/india/topics/poliomyelitis/acute_flaccid_paralysis_surveillance.pdf
15. Polioviruses and other enteroviruses isolated from faecal samples of patients with acute flaccid paralysis in Australia, 1996–2004
    https://www.ncbi.nlm.nih.gov/pubmed/16737480
16. Flaccid Paralysis of Acute Onset in Children (paywalled)
    https://www.ncbi.nlm.nih.gov/pubmed/5437598
17. ACUTE ANTERIOR POLIOMYELITIS IN AN ADULT DUE TO INFECTION
    https://jamanetwork.com/journals/jama/article-abstract/477446
18. Infectious causes of acute flaccid paralysis
    https://www.ncbi.nlm.nih.gov/pubmed/14501988
19. Acute Flaccid Paralysis and Enteroviral Infections
    https://www.ncbi.nlm.nih.gov/pubmed/29959591
20. Acute Flaccid Paralysis | Queensland Health Guidance
    https://www.health.qld.gov.au/cdcg/index/AFP
21. INFANTILE PARALYSIS AN INFECTIOUS DISEASE
    https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(01)50359-1/fulltext
22. About Acute Flaccid Myelitis
    https://www.cdc.gov/acute-flaccid-myelitis/about-afm.html
23. Longitudinal Outcomes in the 2014 Acute Flaccid Paralysis Cluster in Canada: A Nationwide Study
    https://www.ncbi.nlm.nih.gov/pubmed/28193112
24. Field Guide Surveillance of Acute Flaccid Paralysis
    Child Health Division Department of Family Welfare Ministry of Health & Family Welfare New Delhi.
    Prepared with assistance from National Polio Surveillance Project – India | 18 November 2005.
    http://www.searo.who.int/india/topics/poliomyelitis/Field_guide_for_Surveillance_of_Acute_Flaccid_Paralysis_3rd_edition.pdf
25. Enterovirus D68 and acute flaccid myelitis—evaluating the evidence for causality
    https://www.ncbi.nlm.nih.gov/pubmed/29482893