Polio: Understanding, Preventing and Combating this Viral Disease

Poliomyelitis is an acute infection caused by poliovirus, an RNA enterovirus that primarily targets motor neurons in the central nervous system, destroying cells in the anterior horn of the spinal cord or brainstem.  

This disease mainly affects children under five years of age and is most often transmitted by the fecal-oral route. It can lead to acute flaccid paralysis and, without prompt treatment, death from respiratory failure.  

History and Discovery

In 1949, Enders, Weller, and Robbins successfully cultivated the poliovirus in the laboratory, paving the way for vaccines. In 1955, Jonas Salk developed the inactivated poliovirus vaccine (IPV) and tested it on 1.6 million children, reducing cases in the United States by 90%. In 1961, Albert Sabin introduced the oral poliovirus vaccine (OPV), which was more effective at interrupting viral transmission in community settings.

Since the development of doses of Jonas Salk’s inactivated vaccine in 1955, and then of the oral form by Albert Sabin a few years later, vaccination has reduced cases by more than 99% since 1988, with only two countries (Afghanistan and Pakistan) having reported cases of wild poliovirus in 2024. The global effort coordinated by the Global Polio Eradication Initiative (GPEI) relies on mass vaccination (IPV and OPV), wastewater surveillance, and maintaining vaccination coverage. Finally, approximately 25–40% of former paralyzed patients develop, 15–40 years later, post-polio syndrome, characterized by muscle fatigue and chronic pain. Polio can lead to irreversible paralysis and even death in the most severe cases.

Good to know : Children under five are most vulnerable, but anyone who is not immune can be infected.

The main facts

The World Health Organization reports the following facts related to polio:

• Polio mainly affects children under five years old;

• One in 200 infections leads to irreversible paralysis;  

• 5 to 10% of people with paralytic polio die from paralysis of the respiratory muscles;

• Only two countries are still endemic to this day, as the number of cases due to wild poliovirus has decreased by 99% since 1988.

• As long as a single child remains infected, children in every country are at risk of contracting polio. If the disease is not eradicated, it is likely to resurge globally.

Epidemiology and transmission  

In 1988, the World Health Organization (WHO) and its partners launched the GPEI, with a clear goal: to eradicate polio globally. To date, two of the three wild poliovirus serotypes (types 2 and 3) have been declared eradicated, leaving only the wild poliovirus type 1 (WPV1) strain remaining.  

In 2024, there were fewer than 200 cases of WPV1, compared to 350,000 in the late 1980s. Meanwhile, strains of vaccine-derived poliovirus (VDPV) continue to circulate in areas where oral vaccination coverage is insufficient.

Indeed, in 2024, Afghanistan reported 23 cases and Pakistan 39 cases of WPV1, with a marked increase compared to 2023, and positive environmental samples indicating hidden transmission. In addition, vaccine-derived poliovirus (cVDPV) strains continue to circulate in several regions of Africa and Asia, requiring targeted campaigns with monovalent vaccines.

The virus is mainly transmitted by:  

• orofecal route, that is, through the ingestion of contaminated water or food;

• direct contact with an infected person, particularly via respiratory secretions.

Indeed, the polio virus multiplies in the pharyngeal mucosa and in the small intestine. It can be found in the throat and stool. Its transmission is exclusively human-to-human and occurs mainly through the fecal-oral route, particularly through contaminated water and food contaminated by stool.  

Infected people can transmit the infection as long as the virus persists in the throat (one week) and in feces (3 to 6 weeks or longer).

Global Polio Eradication Initiative (GPEI)  

Established in 1988, the GPEI brings together WHO, UNICEF, Rotary, the CDC, the Gates Foundation, and Gavi. By 2024, VDPV cases had fallen by 40% compared to 2023, demonstrating significant progress. However, clusters in conflict settings are hampering eradication.  

Indeed, the main challenges encountered by the GPEI include:  

• conflict zones weaken vaccination coverage and surveillance;

• mistrust has increased since post-COVID-19, which leads to a decrease in vaccine acceptance;

• GPEI faces a deficit of several hundred million dollars, which threatens the countryside.  

After certification of eradication, GPEI plans to:

• Gradually stop the OPV vaccine

• Maintain the IPV to prevent possible reintroduction.
• continuously monitor wastewater and paralysis.

Polio can present in several forms.

Asymptomatic and non-paralytic forms

• Asymptomatic form  : 72 to 95% of infections do not cause any clinical signs, because the virus replicates in the intestine without crossing the blood-brain barrier.

• Non-paralytic form  : 4 to 8% of patients develop viral dysentery  : fever, headache, nausea and vomiting, neck stiffness and muscle pain, without paralysis.

Paralytic form

Approximately 1% of cases progress to acute flaccid paralysis, which typically affects the lower limbs. Paralysis occurs rapidly, often within 24 hours, and may be asymmetrical:

• Spinal paralysis  : damage to spinal motor neurons, mainly lower limbs.

• Bulbospinal (bulbar) palsy  : damage to the brainstem, life-threatening if the respiratory and swallowing muscles are affected.

Postpolio syndrome

Decades after the initial infection, some people develop postpolio syndrome (PPS)  , which manifests as:

• increased fatigue ;

• progressive muscle weakness;

• pain and atrophy;

• breathing or swallowing problems.

PPS management includes pacing techniques and appropriate physiotherapy to avoid overuse of the muscles.