Polio is a very contagious viral disease caused by the poliovirus. It can cause permanent disability and even death, especially in children under the age of five.
There are two types of polio present in the world today: Wild poliovirus and vaccine-derived poliovirus. The latter originates from an oral polio vaccine, known as the Sabin vaccine, or OPV.
Wild poliovirus has been eradicated in most countries, aside from Afghanistan and Pakistan. Vaccine-derived poliovirus has been found in Yemen and central Africa.
Both the wild and the vaccine-derived forms have three types of viruses — types 1, 2 and 3.
Although vaccine-derived polio can take the form of any of the three types, type 1 is the only remaining form of wild poliovirus.
Types 2 and 3 were declared eradicated in 2015 and 2019.
Although all wild poliovirus types can cause the same symptoms, there are differences in how damaging they can be, and immunity to one type does not protect against the other types.
What are the symptoms?
Most people infected with polio are asymptomatic. Around one in four people experience flu-like symptoms, such as a sore throat, fever, headaches or stomach pain. Generally, these symptoms go away on their own after two to five days.
Although rare, a very small percentage — less than 1% — of people infected with the poliovirus experience very dangerous symptoms, such as permanent paralysis, which can lead to a permanent disability, and even death when the virus affects the muscles required for breathing.
Sometimes children that seem to have fully recovered can later, as adults, develop post-polio syndrome, which is characterized by new muscle pain, weakness or even paralysis.
How is polio transmitted?
The virus infects a person's intestines and throat. It can survive there for many weeks. It spreads through a community via contact with an infected person's respiratory droplets or feces.
In places with poor sanitation, the virus can also contaminate food and drinking water.
Infected people can spread the virus to others right before symptoms appear and until up to two weeks after.
Where is polio present today?
Polio has not yet been eradicated worldwide — the wild form of the virus still exists in Afghanistan and Pakistan. And although Africa has been considered free of wild polio since August 2020, imported cases were reported in Malawi and Mozambique .
In July 2022, the US reported its first case of vaccine-derived polio in a decade. Vaccine-derived poliovirus was also detected in sewage samples in the UK and Israel. It was a "stark reminder that if we do not deliver our goal of ending polio everywhere, it may resurge globally," said WHO Director-General Tedros Adhanom Ghebreyesus at the time.
More than a hundred countries have been declared polio-free thanks to the development of polio vaccines in the middle of the 20th century and aggressive global innoculation campaigns.
What are the two types of polio vaccine?
There is no cure for polio, but there are vaccines for preventing the disease: The oral polio vaccine (OPV) and inactivated polio vaccine (IPV).
The oral polio vaccine is administered as an oral liquid and has been key for international eradication because it protects the individual and stops the virus from spreading. The OPV uses live but weakened forms of the poliovirus that are modified not to cause disease in the person taking the vaccine.
But if the OPV-weakened virus is able to stay alive and circulate in places with poor sanitation, such as in wastewater, where there is a high number of unvaccinated people, it can mutate back into a disease-causing form of the virus.
The inactivated polio vaccine is given as an injection and is extremely effective at protecting the receiver from serious disease. Because it's inactivated, it cannot cause vaccine-derived poliovirus. However, unlike the OPV, it doesn't stop the spread of the virus if the person is already infected.
The OPV is cheaper than IPV and does not need to be administered by a health professional. But more and more countries are using IPV, because of the risk of vaccine-derived poliovirus.
There are some types of care that can help with symptoms caused by polio, like bed rest, painkillers, breathing assistance and physical therapy.