As society marks World Polio Day on Monday, this debilitating and sometimes fatal disease is tantalisingly close to being eliminated.
However, recent outbreaks demonstrate that consigning it to the history books will be no easy task.
The disease remains endemic in Pakistan and Afghanistan, where instability and resistance to vaccination programmes make it harder to control.
Meanwhile, polio has recently re-emerged in developed nations, with New York recently recording the first case in the US for nearly 10 years.
“Although the vaccination campaign has pushed it to very low numbers, it certainly hasn’t been eradicated,” said Prof Ian Jones, a professor of virology at the University of Reading in southern England, who researches new approaches to polio vaccination.
Poliomyelitis, to use the full name, is an infectious viral disease mostly affecting young children. In a minority of cases, it causes paralysis and even death, although highly effective vaccines have massively reduced the condition’s global impact.
Vaccine paved way for progress
After polio became a greater public health threat in western nations in the first half of the 20th century, when improved sanitation had limited people’s exposure to the virus and reduced their immunity, the inactivated polio vaccine was introduced in 1955.
Given by injection, this vaccine was followed in 1961 by the oral polio vaccine, which is cheaper and easier to roll out, being administered as drops in the mouth.
By 1988, the threat of polio had fallen away significantly in western nations but, with the disease still endemic in 125 countries, there were about 350,000 cases caused by wild poliovirus, according to the World Health Organisation.
New drive to eliminate disease
The Global Polio Eradication Initiative was launched that year. Involving the deployment of both vaccines, it has also entailed surveillance campaigns to detect the disease and immunisation drives when outbreaks have happened.
Huge progress has been made: last year, the wild poliovirus remained endemic only in Pakistan and Afghanistan, and wild poliovirus resulted in just six reported cases.
In these nations, efforts are hampered by poor healthcare infrastructure and anti-vaccine sentiment, with health workers and the police guarding them having been murdered by extremists.
Separately, this year polio has been detected in cities in developed nations, including in New York, London and Jerusalem.
The pathogen that has been detected is not the wild poliovirus found in Pakistan and Afghanistan, but instead it is circulating vaccine-derived poliovirus (cVDPV).
This is a virus that derives from the oral polio vaccine, which contains a weakened or attenuated form of poliovirus that can sometimes mutate into a form that causes disease.
Among populations with low immunisation rates, cVDPV can spread from person to person, and in rare instances cause disease.
In Israel, which administers both the inactivated polio vaccine and the oral polio vaccine, an unvaccinated child of 3 in Jerusalem suffered paralysis because of cVDPV in a case reported to the WHO in March.
A few months later in the US, an unvaccinated man in Rockland County, just north of New York City, became infected and suffered paralysis.
New York’s governor declared a state of emergency last month, when monitoring of wastewater samples — the virus can be detected in the faeces of infected individuals — indicated cVDPV was still present.
It has also been detected this year in wastewater samples in London.
Both the US and the UK use a form of inactivated polio vaccine, so experts say cVDPV is likely to have been brought in from outside each country.
“What it tells us, as it’s vaccine derived virus in the sewage waters, is that someone in the US or London has been in a part of the world where they still use live attenuated virus,” said Dr Bharat Pankhania, a senior consultant on communicable disease control and senior clinical lecturer at the University of Exeter in south-west England.
“The risk is low because most people in developed parts of the world are already immune … and I marvel at our ability to find it in wastewater. It’s a new addition that can be used to pick up things and act on it if we have to.”
UK vigilant over threat
The UK’s National Health Service has said the number of children vaccinated against polio in London “is lower than it should be”, which could aid the virus’s spread. In response, officials are redoubling vaccination efforts.
Other outbreaks of cVDPV, such as one in Syria in 2017 and one in Somalia in 2018 — which resulted in seven children becoming paralysed — were eliminated through surveillance and vaccination.
Genetic analysis has found links between the cVDPV in the US, UK and Israel, potentially suggesting widescale circulation of the virus over time, which may mean outbreaks will be harder to control.
Despite the enormous strides made in reducing cases, experts say that it is not a given that the wild poliovirus will be eliminated.
Prof Jones says that eradication is not impossible scientifically. Unlike some other diseases, polio does not have an animal “reservoir”, meaning that there is no animal that continues to harbour the parasite and risks spreading it to people.
However, political turmoil that disrupts efforts to stamp out the disease will continue to affect certain parts of the world, Prof Jones said, and in these areas polio is likely to remain.
“There will always be pockets where the vaccine coverage is not what it should be and there will be breakouts,” he said.
In these areas there is often a “helicopter” approach of blanket vaccination, but Prof Jones said the disease typically then “pops up somewhere else”.
“I personally think control to keep it at the minimum level possible is the most realistic outcome. I don’t think eradication is going to happen any time soon,” he said.