Poliomyelitis, or polio, is not eradicated, but global efforts have brought it to the verge of elimination. This infectious viral disease primarily affects children under five and can lead to irreversible paralysis. The virus spreads person-to-person through contact with contaminated feces, especially in areas with poor sanitation. While there is no cure, polio is preventable through vaccination. Since the launch of the Global Polio Eradication Initiative (GPEI) in 1988, cases have been reduced by over 99.9%. The world is close to making polio the second human disease, after smallpox, to be eradicated.
Current Global Status of Poliovirus
The fight to eliminate polio has seen success, with wild poliovirus transmission now restricted to just two countries: Afghanistan and Pakistan. This is a dramatic decrease from 1988, when the virus was endemic in more than 125 countries and caused an estimated 350,000 cases of paralysis annually. Coordinated worldwide vaccination campaigns have prevented more than 20 million cases of paralysis.
This achievement remains fragile. The continued presence of the virus in any location means all countries are at risk, as international travel can reintroduce the virus to polio-free areas. The only permanent solution is complete eradication. Until transmission is stopped everywhere, no child is safe from polio.
A distinction must be made between the naturally circulating “wild poliovirus” and another form known as vaccine-derived poliovirus. While wild poliovirus is confined to small geographic pockets, outbreaks of vaccine-derived polio occur in other parts of the world. Both types can cause paralysis and must be eliminated to achieve a polio-free world.
Eradication Progress by Virus Type
The poliovirus exists in three wild-type strains, and progress against them has been methodical. Wild poliovirus type 2 (WPV2) was certified as globally eradicated in September 2015, with its last detection occurring in India in 1999. This milestone proved that permanent elimination of a polio strain was achievable.
In October 2019, wild poliovirus type 3 (WPV3) was also declared eradicated, with the last case recorded in Nigeria in 2012. The eradication of two of the three wild strains has simplified the global effort. This allows resources to be concentrated on the final target.
This leaves only wild poliovirus type 1 (WPV1) in circulation. WPV1 is the cause of all wild polio cases today and is the strain transmitted in the border regions of Afghanistan and Pakistan. The elimination of WPV1 will mark the final step in eradicating all wild poliovirus.
Vaccine-Derived Poliovirus Outbreaks
A separate challenge comes from circulating vaccine-derived poliovirus (cVDPV), which is linked to the oral polio vaccine (OPV). The OPV contains a live, but weakened, form of the poliovirus that stimulates an immune response. It has been the primary tool in the eradication campaign.
In rare instances, particularly in communities with low immunization coverage, the weakened virus from the vaccine can circulate. As it passes from person to person over time, it can undergo genetic changes and revert to a form capable of causing paralysis. These cVDPV outbreaks are a consequence of using the oral vaccine in under-vaccinated populations.
These outbreaks are distinct from wild polio and require a specific response. The emergence of cVDPV highlights the necessity of maintaining high vaccination rates to prevent the vaccine virus from circulating long enough to mutate. Responding to these outbreaks, which have occurred in parts of Africa and the Middle East, is a component of the endgame strategy.
The Final Push for Eradication
The Global Polio Eradication Initiative (GPEI), a partnership including the WHO, Rotary International, and the CDC, leads the charge against the virus. A primary element of the strategy involves a transition in vaccine usage. Many regions are supplementing or replacing the oral polio vaccine (OPV) with the inactivated polio vaccine (IPV), which is injected and contains a killed virus, making it incapable of causing vaccine-derived polio.
Robust surveillance systems are another pillar of the strategy. This includes looking for patients with paralysis and conducting environmental surveillance by testing sewage for the poliovirus. This method can detect the virus circulating in a community even if no paralysis cases are reported, providing an early warning for a public health response.
When an outbreak is detected, rapid response teams are deployed to conduct emergency vaccination campaigns. The goal is to boost immunity in the surrounding population and stop the virus from spreading. Implementing these strategies in the last affected areas is the final step to ensuring no child suffers from polio paralysis again.