Polio, medically known as poliomyelitis, is a debilitating viral disease that has been known for thousands of years and occurs most often in children under age 5. Roughly 70% of all polio cases are asymptomatic, and most other cases produce only flu-like symptoms (headaches, nausea, fever)—however, approximately 1%-5% of cases develop much more severe symptoms including meningitis and/or muscle paralysis, which can lead to permanent disability or even death. Those who recover may also experience post-polio syndrome, in which symptoms recur 15-40 years later. Polio is highly infectious and has no cure, nor any specific treatment regimen.
Polio was a major concern in the United States in the 1950s, where 20,000 people were inflicted with paralytic poliomyelitis in 1952 alone. Fortunately, modern vaccination programs have checked polio's advance in most of the world. While developed countries with well-funded health care networks were the first to benefit from the vaccines, 1988 saw the launch of the Global Polio Eradication Initiative, which sought to immunize developing and low-income countries against the disease as well. Since 1988, global poliovirus cases have fallen by 99.9%, and only one of the original three types of wild poliovirus (WPV1) remains in circulation.
Today, polio is endemic in only two of the world's countries: Afghanistan and Pakistan. Even in these locations, aggressive vaccination efforts have dropped the number of cases to a handful a year. That said, both these two countries and many others as well, particularly in Africa and Asia, are still vulnerable to outbreaks if the disease is accidentally introduced by international travelers or some other means.
Number of Wild Poliovirus Cases in Endemic Countries 2016-2022*
*Data for 2022 is partial-year data recorded from 01 January to 13 September.
What is vaccine-derived poliovirus (cVDPV) and what is acute flaccid paralysis (AFP)?
The primary polio vaccine in many parts of the world (the United States excluded) is the oral polio vaccine (OPV), which creates immunity in its patient by introducing a small dose of highly weakened, but still living poliovirus. This techinique is both safe and common in the medical field, and OPV can be administered with a few drops of liquid on the tongue, with no need for a hypodermic needle—a major advantage in terms of cost, training, and logistics. However, in under-immunized areas in which some people are vaccinated and others are not and the weakened poliovirus can circulate for a year or more, it can to mutate into a more virulent form known as circulating vaccine-derived poliovirus (cVDPV).
CVDPV often goes undetected until an extreme case causes a condition known as acute flaccid paralysis (AFP) in an infected individual. Although AFP can also be caused by a non-polio-related affliction, every case of AFP is closely analyzed to determine if it might signal a polio outbreak. Fortunately, cVDPV is quite rare and can be controlled using the same techniques as wild polio (WPV1).
Countries in which polio is still a major concern
Pakistan is one of two countries in which polio is still endemic, but eradication seems to be close. 17 cases of wild polio were recorded in the country during the first eight months of 2022, compared to 84 total in 2020. The 2020 COVID-19 restrictions delayed immunization efforts in countries with polio, including Pakistan, and likely contributed to the country's 138 cases of AFP in 2020 (second-most in the world). However, efforts had resumed in earnest by 2022, with millions of children receiving immunizations in January alone. Immunization efforts continued throughout the year, although excessive flooding in late 2022 introduced complications and was expected to increase the risk of polio transmission.
Global Polio Eradication has a plan to provide vaccines to all children in Afghanistan, the second remaining country where polio is endemic. The frequency of confirmed cases of wild polio in Afghanistan plummeted from 54 cases spread across 38 infected districts in 2020 to only 4 cases spread across 2 districts in 2021. Afghanistan led the world in AFP from vaccine-derived poliovirus in 2020 with 308 cases.
One of 84 high-risk districts, the UAE announced in July 2021 that it would fund 16 million child vaccinations. Global Polio Eradication agreed to allocate about $9.5 million for this project.
No new wild poliovirus cases have been reported in Nigeria since 2016, despite the fact that as recently as 2012, half of worldwide polio cases occurred in Nigeria. The country was declared polio-free in 2020, and although the lack of polio vaccines during the COVID-19 pandemic and quarantine was a significant concern, no new cases of wild polio were reported during that time. Nigeria did, however, record a massive 415 cases of AFP in 2021, signaling that there is still work to be done.
National Immunization Days returned in 2005 to reach some of the communities most affected by conflict and poverty in Sudan, one of the world's poorest countries. The UN Children’s Fund and World Health Organization also participated in this immunization effort. While conditions have improved in Sudan, instances of vaccine-derived poliovirus spiked in 2020, with 58 cases of AFP, possibly as a result of the COVID-19 quarantine.
Sudan's neighbor Chad, is another polio-free country in which cVDPV appears to have risen in 2020, with 101 cases of AFP. Rates of AFP dropped to zero in 2021 before rising again to 18 through the first 8 months of 2022, according to The World Health Organization.
Like many countries, Cameroon was impacted by the WHO decision to temporarily interrupt polio vaccination campaigns in 2020. However, the country also recorded only 7 cases of AFP in 2020 and 3 in 2021, so it seems to have recovered nicely.
During the 2020 Covid-19 pandemic, Angolan children could not access the immunizations they needed. This shortage is arguably the main reason the country registered 138 cases of AFP druing 2020. However, immunizations resumed by the end of the year and only 3 cases of AFP were recorded during 2021.
9. DR Congo
More than any other country, the Democratic Republic of the Congo has struggled with vaccine-derived poliovirus (cVDPV). DR Congo has registered numerous cases of AFP every year for more than half a decade: 22 in 2017, 20 in 2018, 88 in 2019, 81 in 2020, 28 in 2021, and 110 and counting in 2022. Its sister country, Republic of the Congo, has recorded a total of 4 cases during that entire time.