Polio is a highly infectious disease caused by the poliovirus, an enterovirus that attacks the nervous system. Massive vaccination campaigns led to its near-eradication worldwide, and for decades, many regions were declared polio-free. However, recent detections of the virus in unexpected locations suggest that this progress is fragile. These findings indicate a heightened risk of a global resurgence, even in areas that have not seen a local case in generations.
Understanding the Poliovirus
The poliovirus is an RNA virus that primarily enters the body through the mouth, most often via the fecal-oral route (contaminated food, water, or hands). The virus multiplies in the throat and intestines, where most infections are contained. Most people (70% to 95%) show no symptoms or only mild, flu-like symptoms, making the virus a silent spreader capable of circulating undetected.
In less than one percent of infections, the virus travels from the bloodstream to the central nervous system. It targets and destroys motor neurons in the spinal cord and brain stem. This destruction results in acute flaccid paralysis, which is often permanent and typically asymmetrical. In severe instances, paralysis can affect the muscles needed for breathing, leading to death in 5% to 10% of paralytic cases.
Indicators of Resurgence
The primary signal of resurgence is the continued endemic circulation of wild poliovirus type 1 in Pakistan and Afghanistan. As long as the virus survives there, it can be exported to any country with low vaccination rates. Recent evidence of community transmission in areas long considered polio-free suggests the virus is finding pockets of susceptible people.
Public health officials have identified the virus in wastewater samples across major global cities, including New York and London. Detecting poliovirus in sewage confirms the virus is actively circulating and being shed by infected individuals. This environmental surveillance serves as an early warning system, revealing silent transmission before a paralytic case is identified. In New York, this surveillance followed the 2022 confirmation of a paralytic polio case in an unvaccinated adult, the first U.S. case in nearly a decade. The virus found in the New York patient and the London wastewater samples was genetically linked, highlighting its ability to spread internationally and re-establish itself in undervaccinated populations.
The Role of Polio Vaccines
Two primary types of polio vaccines are used globally, each with a distinct mechanism of action.
Inactivated Poliovirus Vaccine (IPV)
The IPV is given as an injection and contains killed poliovirus strains. IPV is highly effective at preventing paralytic disease by generating antibodies in the bloodstream. However, it does not prevent the virus from multiplying in the gut and being shed in stool.
Oral Poliovirus Vaccine (OPV)
The OPV contains a live, weakened form of the virus and is administered as drops. OPV provides superior local immunity in the gut, which blocks the virus’s initial replication and prevents transmission to others. In extremely rare cases, the live, weakened virus in the OPV can change genetically if it circulates in under-immunized populations for an extended period. This mutated version, known as a circulating Vaccine-Derived Poliovirus (cVDPV), regains its ability to cause paralysis. The cVDPV is responsible for the recent outbreaks in New York and London.
Personal Prevention and Community Immunity
The most effective action an individual can take is to ensure their vaccination status is current. For children, the recommended schedule involves four doses of the IPV. This series is typically administered at:
- Two months
- Four months
- Six to eighteen months
- A final booster between four and six years of age
Vaccination protects the individual from developing paralysis and contributes to community immunity, also known as herd immunity. This collective protection occurs when a sufficiently high percentage of the population is immune, making the chain of transmission difficult to maintain. Health experts estimate the community-wide vaccination threshold must remain above 80% to prevent sustained transmission. Recent virus detection in areas with local vaccination rates falling below this threshold highlights how pockets of unvaccinated individuals create vulnerability for the virus to return and spread silently.