Poliomyelitis, commonly known as polio, is a highly infectious viral disease that historically caused devastating outbreaks of paralysis, primarily in children under the age of five. The global effort to eliminate this disease began in 1988, dramatically reducing case numbers by over 99% worldwide through massive vaccination campaigns. While this public health initiative has been highly successful, the disease is not yet fully “eradicated.” The current status involves fighting the naturally occurring Wild Poliovirus and addressing a separate challenge posed by vaccine-related forms of the disease.
Eradication vs. Elimination: Understanding the Distinction
The terms “eradication” and “elimination” carry distinct meanings within public health. Elimination is achieved when the incidence of a disease is reduced to zero in a specific geographic region, meaning it no longer naturally circulates there. However, an eliminated disease can return if the pathogen is re-introduced, requiring continued intervention and surveillance.
Eradication, by contrast, refers to the permanent reduction to zero of the worldwide incidence of infection caused by a specific agent. This is the complete and final removal of the pathogen from the planet, meaning no further intervention measures are required anywhere. Only one human disease, smallpox, has ever been officially declared eradicated, which highlights the immense scale of the goal the global polio initiative is pursuing.
The Current Status of Wild Poliovirus
The fight against the naturally occurring Wild Poliovirus (WPV) has seen extraordinary success, with two of the three strains officially removed from circulation. WPV Type 2 was certified as globally eradicated in 2015, and WPV Type 3 was certified as eradicated in 2019. This leaves WPV Type 1 as the sole remaining wild strain still circulating globally.
Endemic WPV Type 1 transmission is now confined to just two countries: Afghanistan and Pakistan. This region presents a single epidemiological block where the virus persists, often along shared border areas. Geopolitical instability, security challenges, and difficulty accessing children in remote or conflict-affected zones create substantial logistical hurdles for vaccination teams. Failure to interrupt transmission in these two countries puts every other country at risk of re-importation, as seen when WPV1 linked to Pakistan was detected in Malawi and Mozambique in 2021 and 2022.
The Challenge of Vaccine-Derived Poliovirus
A challenge to eradication comes from Circulating Vaccine-Derived Poliovirus (cVDPV), which arises from the use of the Oral Polio Vaccine (OPV). The OPV contains a live, weakened form of the virus that provides intestinal immunity and is excreted by the vaccinated person.
In populations with low vaccination rates and poor sanitation, this excreted vaccine virus can spread. As the weakened virus circulates and replicates, typically over 12 to 18 months, it can genetically mutate and regain the ability to cause paralysis. This newly virulent cVDPV can then spread like the wild virus, leading to outbreaks; cVDPV Type 2 is the most prevalent form.
In 2022, poliovirus linked to cVDPV was detected in wastewater in developed countries like the United States and the United Kingdom, underscoring the risk of silent spread in under-immunized areas. To counter this threat, the novel Oral Polio Vaccine Type 2 (nOPV2) has been developed and deployed under emergency use authorization. The nOPV2 is genetically engineered to be significantly more stable than the traditional OPV, making it much less likely to mutate into a disease-causing form. Since its introduction in 2021, billions of doses of nOPV2 have been administered globally to respond to cVDPV2 outbreaks.
Global Surveillance and Maintaining Polio-Free Status
The vast majority of the world maintains a polio-free status, upheld by constant global surveillance efforts. The primary method for monitoring poliovirus is Acute Flaccid Paralysis (AFP) surveillance, which involves investigating every case of sudden-onset flaccid paralysis in children under 15 years of age. Stool samples are collected and analyzed to determine if the paralysis is caused by WPV, cVDPV, or another pathogen.
To supplement this case-based system, environmental surveillance, often called wastewater testing, has become a valuable tool. Poliovirus is shed in the feces of both symptomatic and asymptomatic individuals, meaning testing sewage samples can detect the circulation of the virus before any cases of paralysis appear. This early warning system identifies silent circulation, allowing public health officials to rapidly deploy vaccination campaigns. Maintaining high routine immunization coverage remains the most effective defense against all forms of the virus.