Polio is on track to become the second human disease ever to be eradicated, with a global decline in cases of 99% since 1998. Of the three strains of wild poliovirus, only one – wild poliovirus type 1 — remains in circulation.
However, low vaccination rates are cause for “global concern,” according to Dr. Zubair Mufti Wadood, a senior epidemiologist working in the polio department at the World Health Organization (WHO).
Poliomyelitis (polio) is a highly infectious viral disease that spreads from person to person, mainly by fecal-oral transmission or, less commonly, from droplets from a sneeze or cough of an infected person or by a common vehicle (e.g. contaminated water or food).
While most cases of polio are asymptomatic, the virus can invade a person’s nervous system and cause paralysis and, in some cases, death. Risk factors include poor sanitation, high population densities, and low immunization rates.
Some populations are at greater risk of contracting polio due to conflict, insecurity, or weakened health infrastructure — elements that lead to low immunization rates. High-risk population subgroups also include populations in hard-to-reach areas and nomadic groups.
The polio vaccine, which is administered orally (OPV) or through a shot (IPV), can protect a child for life.
But in areas with low vaccination rates, population immunity is weak and unable to stop transmission, which creates a risk of an international spread, as well as vaccine-derived poliovirus.
Since 2017, the world has seen more paralytic cases of polio due to vaccine-derived polioviruses than wild poliovirus, WHO’s Wadood told Global Citizen — which is a direct result of low vaccination rates.
The oral polio vaccine, which provides lifelong protection from polio, contains a weakened form of the poliovirus, which helps children develop immunity against the disease. A child cannot be infected with polio from receiving the vaccine. In rare cases among under-immunized populations, unvaccinated children may be at risk of vaccine-derived polioviruses.
In communities with low vaccination rates and poor sanitation, however, the live weakened virus that is contained in the oral polio vaccine may be allowed to circulate for long periods. Over time, the weakened virus can mutate and revert to a variant form that can cause paralysis, called vaccine-derived poliovirus, also known as VDPV. In some instances, the virus continues to spread through human-to-human transmission, or community transmission, which is referred to as circulating vaccine-derived poliovirus, also known as cVDPV.
According to Wadood, a vaccination rate of over 90% among children is sufficient to prevent the emergence of VDPV.
Lower immunity of a population leads to the virus surviving longer, which in turn, enables it to replicate, change, and spread more from one unvaccinated child to another. The VDPV viral strains can cause paralysis, as well as outbreaks. However, if a population is fully immunized against polio, it is protected against both the wild poliovirus and VDPV.
According to Wadood, who supports planning and implementing polio eradication programs at the WHO, the name “vaccine-derived poliovirus” is misleading — causing some people to believe that the virus comes from the vaccine itself.
“The vaccine-derived poliovirus is by no means a vaccine side effect or a vaccine consequence. It entirely depends upon the population immunity and the vaccination or immunization coverage in a given area,” Wadood told Global Citizen.
If vaccination rates are high and population immunity is high as a result, the virus cannot mutate, which means cVDPV will not emerge and cause an outbreak.
Ultimately, “the higher the vaccination coverage, the [smaller] the risk of cVDPV outbreaks,” he explained.
To date in 2022, cases of cVDPV resulting in paralysis have been reported in Niger, the Democratic Republic of Congo (DRC), Nigeria, Somalia, Madagascar, Mozambique, Chad, Togo, and Yemen. In 2021, there were cases in 24 countries including Ukraine, Afghanistan, and Tajikistan.
In 2020, due to several factors, including pausing routine vaccination campaigns during the COVID-19 pandemic lockdowns, paralytic cases of vaccine-derived polio nearly tripled from the previous year, as over 1,000 paralytic cases were reported globally. Since 2020, paralytic cases have declined, with fewer than 700 cases reported in 2021, and an additional decline in 2022 to date, with a total of 158 cases as of July 2022.
Areas of particular concern are the DRC, Yemen, Somalia, and northern Nigeria, where vaccine coverage is low and which accounts for approximately 80% of global cVDPV cases.
In some Nigerian states, such as Sokoto and Zamfara, polio vaccine coverage in routine immunization was as low as less than 10% in 2021, Wadood said. As a result, more than 80% of the new cVDPV cases in Nigeria are reported from states that have less than 40% IPV coverage in routine immunization, he explains.
The good news, he shares, is that the virus is geographically concentrated and is decreasing annually. However, Wadood also said these outbreaks have been persistent, and “puts all the other countries in the world at a very high risk of falling prey to cVDPV outbreaks.”
In June, the Global Polio Laboratory Network confirmed vaccine-derived poliovirus from environmental samples taken in London, UK. There have been no cases of paralysis detected as the virus was found in samples. Currently, the virus is not deemed to be circulating, nor is there an official outbreak.
While case numbers may seem low, even a single case in a child is dangerous as it puts children everywhere at the risk of contracting polio. According to the WHO, failure to eradicate polio could result in as many as 200,000 new cases every year.
The GPEI has been working to eradicate polio worldwide since 1988. It is a partnership of national governments, the WHO, Rotary International, the US Centers for Disease Control and Prevention, UNICEF, the Bill & Melinda Gates Foundation, and Gavi, the Vaccine Alliance.
GPEI estimates that their work has prevented 20 million people from being paralyzed and 1.5 million people from dying as a result of polio.
In April, GPEI launched an investment case, calling for countries and partners to commit the $4.8 billion needed to implement its 2022-2026 strategy, which would enable the delivery of vaccinations for 370 million children annually for the five-year period and fund global surveillance activities for polio and other diseases.
Circulating vaccine-derived polio poses a threat to the eradication of polio globally. In response, it is essential to strengthen polio surveillance systems and ensure high vaccination coverage. This would prevent cases of both wild poliovirus and cVDPV, and would ensure that no child in the world is at risk of contracting this infectious disease.
In addition, the GPEI is rolling out a new vaccine, the novel oral polio vaccine type 2 (nOPV2), that could help to more sustainably stop outbreaks of type 2 cVDPVs.
“The modifications made in nOPV2 allow it to be just as effective at stopping outbreaks as the OPV already in use, but much less likely to revert to a form that can cause paralysis,” Dr. Ananda Bandyopadhyay, deputy director for polio at the Bill & Melinda Gates Foundation, said in a statement issued to Global Citizen. “This new vaccine can make a real difference in how we prevent cVDPVs and stop polio transmission for good.”
The end of polio is in sight, but the virus is trying its best to stage a revival. The Comeback We Never Wanted is a content series that looks at how and why polio outbreaks have increased in recent years, diving into issues connected to access to health care, touching on the impact of COVID-19, the difficulty of navigating conflict zones and terrorist groups, and more.
Disclosure: This series was made possible with funding from the Bill & Melinda Gates Foundation.
Editor's note: A previous version of this article misstated the polio vaccination rates in some Nigerian states. It has been updated with the correct statistics from 2021.