- Polio, an infectious disease that can cause paralysis, is endemic in just two countries: Afghanistan and Pakistan.
- The development of polio vaccines in the mid-twentieth century and a robust international response have helped more than one hundred countries become polio-free.
- New cases in the United States and Israel in 2022 underscore the risk that, until it is eradicated globally, polio can make a comeback in any part of the world.
Once endemic in more than one hundred countries, polio today has a foothold in just two: Afghanistan and Pakistan. At its peak, the infectious disease killed or paralyzed hundreds of thousands of people annually, but breakthroughs in vaccines and public health programs have made remarkable progress. A 2014 decision by the World Health Organization (WHO) to declare it an international health emergency sought to buttress the global response, and the designation remains in place as health agencies work to eradicate the disease.
However, there are challenges to vaccinating the world’s remaining unprotected children: some communities strongly oppose vaccination efforts, and polio workers have come under attack by militants. Underscoring the dangers, polio reemerged in southern Africa in 2021, and cases of vaccine-derived polio surfaced in Israel and the United States in 2022. Meanwhile, the war in Ukraine has disrupted efforts to manage an outbreak in the country’s west.
What is polio?
Poliomyelitis, or polio, is a highly infectious viral disease that mostly affects young children. Poliovirus is most often transmitted by sewage-contaminated drinking water. It can survive for two months outside the body and is particularly transmissible in areas with open sewers. About three-fourths of infections are asymptomatic, allowing the disease to spread undetected. When the virus reaches the brain and spinal cord, it can result in permanent paralysis or death.
Since there is no cure for polio, public health officials have focused on mass immunization to control the disease. Two polio vaccines are widely used: the inactivated polio vaccine (IPV) developed by Jonas Salk in 1954, which is administered through injection and gives immunity through one dose, and the oral polio vaccine (OPV) developed by Albert Sabin in 1961, which contains live strains of the virus and is more easily administered than the Salk vaccine. Both vaccines lose efficacy when exposed to high temperatures and therefore need to be kept cool. The OPV requires several boosters to be effective.
In addition to wild poliovirus, there is vaccine-derived poliovirus, which can emerge when the weakened strain in an OPV mutates in unvaccinated or undervaccinated communities to behave more like the wild virus. Today, it is the most common form of poliovirus.
How successful have eradication efforts been?
In 1955, Salk announced the successful results of a large-scale test of the first polio vaccine, and by 1957, a mass immunization campaign had reduced the U.S. infection rate by 90 percent. By the time Sabin developed the orally administered vaccine four years later, there were only 161 recorded cases of polio in the United States, compared to more than twenty thousand paralytic cases in 1952. The country was declared free of the disease in 1979.
Countries with advanced health systems integrated polio immunization into pediatric care, but developing countries took campaign approaches, mobilizing vaccination workers who would inoculate thousands of children in a single day. Rotary International, a U.S.-based nongovernmental organization (NGO), played an important role in organizing many of these campaigns, often alongside governments, the WHO, and smaller NGOs.
Some campaigns were carried out under extremely difficult conditions. For example, even at the height of civil conflicts in El Salvador, Guatemala, and Nicaragua, combatants observed cease-fires to permit mass vaccinations. Typically, the UN Children’s Fund (UNICEF) and local health workers would join forces to administer immunizations. “Until the Taliban and Islamists in Nigeria changed the entire picture, it was the case that polio vaccination was not going to be part of conflict,” said former CFR Senior Fellow Laurie Garrett. “[Polio] had a truly neutral status.”
In 1988, the WHO, having rid the world of smallpox eight years before, launched the Global Polio Eradication Initiative (GPEI) in partnership with Rotary International, UNICEF, and what is now the U.S. Centers for Disease Control and Prevention (CDC). At the time, there were 350,000 cases reported worldwide; that figure fell 99 percent by 2000. The Bill & Melinda Gates Foundation has become a champion of fighting polio, donating billions of dollars [PDF] to eradication efforts and galvanizing support among donor countries and private citizens.
The CDC estimates that vaccination has prevented eighteen million cases of paralytic polio and 1.5 million polio-related child deaths since 1988. India, which as recently as 2002 carried much of the world’s total caseload, was declared polio-free in 2014. A country has to go three years without detecting any new cases for the WHO to make that designation.
The year 2020 marked a notable advancement toward eliminating polio, with the WHO announcing that wild polio had been eradicated in Africa. (Nigeria, the last African country with endemic polio, was declared polio-free in 2015.) However, wild polio reemerged on the continent late the following year, with a paralytic case discovered in Malawi.
Where is polio now?
Polio remains endemic in just two countries: Afghanistan and Pakistan. (Epidemiologists use endemicity to identify places where transmission of poliovirus has never been interrupted.) Of the world’s 110 cases of wild polio in 2020, two-thirds were in Pakistan and the remainder were in Afghanistan. The following year, Pakistan reported just one case of paralytic polio, compared to more than three hundred such cases in 2014.
However, not long after Africa was declared free from wild poliovirus, it has resurfaced in Malawi and Mozambique. And some countries are experiencing upticks in vaccine-derived polio: There have been more than five hundred vaccine-derived cases in the past year, with the vast majority in Nigeria, Yemen, and the Democratic Republic of Congo. The United States reported its first such case in nearly a decade, while Israel saw its first in over thirty years. At the same time, the detection of polio in wastewater in New York and London suggests that the virus is circulating and has prompted catch-up vaccination campaigns.
Additionally, health experts are raising the alarm over the return of polio in Ukraine amid Russia’s invasion in early 2022. Following the discovery of several vaccine-derived cases in late 2021, Ukraine’s health ministry and the WHO set in motion a major vaccination campaign, but it has been interrupted by war. The reemergence of polio often occurs in areas of conflict or instability, where war has disrupted health systems, as well as in areas where refugees from conflicts have resettled. For example, Syria saw a resurgence of the disease in 2013–14 during the country’s civil war. The map of polio, wrote journalist Sarah Stillman in 2014, “is a map of modern political violence.”
Why has eradication proven so difficult?
Polio is extremely contagious and most carriers never show symptoms. For each person who displays paralytic symptoms of polio, there are as many as two hundred others [PDF] with mild or no symptoms who can still transmit the disease. The polio vaccine is also more complicated to administer than some others, such as the smallpox vaccine; the oral polio vaccine needs to be administered several times and kept refrigerated.
Another major challenge to eradication is mistrust of the West and vaccination programs. Despite national governments’ public support for polio eradication efforts, there is strong opposition to vaccines in some afflicted areas. Polio immunization workers have come under attack by militants in endemic areas, with more than seventy workers targeted in Pakistan since 2012. Eight polio vaccination workers were killed in northern Afghanistan in February 2022, leading the United Nations to halt vaccination efforts in two provinces. The following month, gunmen killed a polio worker in northwestern Pakistan.
A ban on polio vaccinations by the Tehrik-e-Taliban Pakistan (TTP) militant group has further hindered eradication efforts in Pakistan. In 2012, a TTP commander prohibited polio vaccinations in North Waziristan (a district of Khyber Pakhtunkhwa Province), and the ban was later extended to South Waziristan. TTP leaders say the ban was in response to U.S. drone strikes; but vaccination efforts were also complicated by a controversial CIA-backed move in which a medical team ran a hepatitis B vaccination campaign in 2011 to gather information on al-Qaeda leader Osama bin Laden’s whereabouts. Health officials said the ruse threatened to “set back global public health efforts by decades,” and, in 2014, the CIA banned the practice of using vaccination programs as cover for its operations. That same year, a surge in cases prompted the WHO to declare a public health emergency of international concern (PHEIC), a designation that remains in place due to the ongoing risk of the virus’s spread across borders.
The Afghan Taliban had banned door-to-door vaccination efforts in 2018 but ended the ban in 2021, shortly after taking over control of Afghanistan and installing a new government. The group is now backing a national vaccination campaign led by the WHO, and aid agencies say that they have access to previously hard-to-reach communities. However, the country remains embroiled in economic and humanitarian crises following the Taliban’s takeover, and the virus can easily be transferred across the border with Pakistan.
Polio vaccination campaigns in countries such as Pakistan and Nigeria have also been undermined by misinformation. In 2019, for example, a mob in Khyber Pakhtunkhwa burned down a health-care facility after rumors spread that recently vaccinated children had fallen ill because their doses were expired. A 2014 Harvard School of Public Health survey [PDF] of caregivers in Nigeria and Pakistan found that between one-quarter and one-third of respondents believed polio paralysis was curable, and between one-fifth and one-third did not know that the oral vaccine needs to be administered multiple times to maximize its efficacy. The poll also found that mistrust of polio workers was higher in conflict-ridden zones such as Khyber Pakhtunkhwa in Pakistan and Nigeria’s Borno State, where the militant group Boko Haram is active. Despite misperceptions, the poll still found low refusal rates for the OPV.
The COVID-19 pandemic has added to the challenge. The spread of the new coronavirus in early 2020 caused an estimated twenty-three million children to miss routine vaccinations as polio vaccination efforts around the world were halted. Both Afghanistan and Pakistan saw spikes in cases that year, though they have since dropped to historic lows with the resumption of immunization campaigns. Experts say they’re optimistic that efforts to eradicate polio will benefit from the increased awareness the pandemic brought to the lifesaving effects of vaccines.
What is being done to eradicate polio?
In response to the paralytic case in Malawi, UNICEF launched a new campaign in 2022 to vaccinate twenty-three million children across the southern African country and its neighbors. With help from the WHO, the Malawian government set up surveillance sites across the country and drew up what it called a “meticulous” plan to reach every child; the CDC is assisting with surveillance.
Meanwhile, the GPEI continues to lead efforts to eliminate polio in Afghanistan and Pakistan. The WHO lauded the successful immunization of 2.6 million children in Afghanistan in late 2021, though officials warned that the 2022 attack on vaccination workers underscores persisting security challenges. In Pakistan, UNICEF and other international agencies have voiced hope that eradication is within reach after impressive progress in recent years. “Being in Pakistan now feels like being in India and Nigeria right before they achieved ultimate success,” the Gates Foundation’s Michael Galway wrote in February 2022, noting that the government and its partners have ramped up data monitoring, social mobilization, and security efforts. However, there remains resistance from communities in parts of Khyber Pakhtunkhwa, the only region where the virus still circulates.
To combat vaccine-derived polio, scientists have been developing a new oral vaccine, known as nOPV2, over the last decade. Clinical trials have shown it to be safe and effective while being more stable, and thus less likely to mutate in underimmunized communities. Distribution of nOPV2 began in 2021, after the WHO granted it an emergency use listing, and more than a hundred million doses have been administered. However, the widening of its availability has been slow, and WHO officials have urged countries to respond swiftly to outbreaks with vaccines at their disposal rather than wait for the novel vaccine.
The GPEI estimates it will cost $5.1 billion to eliminate polio by 2023, which will require the stockpiling of vaccines to sustain eradication. Between 1988 and 2020, more than $18 billion was spent on eradication efforts. Those efforts have already prevented some $27 billion in health costs, according to the GPEI, and it projects that being polio-free will save the world roughly half that amount by 2050. A 2021 study in the journal Risk Analysis put the health and economic benefits of eradication at $28 billion, including productivity gains.
For Think Global Health, the United Nations Foundation’s John E. Lange writes that he is not ready to give up on the goal of eradicating polio.
This timeline looks at major epidemics since the start of the twentieth century.
This Backgrounder looks at the WHO’s role.
The WHO breaks down how vaccines protect against dozens of life-threatening diseases.
For Time, Jeffrey Kluger and Tara Law explain how Russia’s invasion has threatened progress against polio in Ukraine.
In 2020, Rana Jawad Asghar, CEO of Global Health Strategists & Implementers, laid out in the Lancet why polio is still a problem in Pakistan.
Antonio Barreras Lozano contributed to this Backgrounder. Will Merrow helped create the graphics.