Polio: How dangerous is the disease and how close is it to being eradicated globally?

The discovery of traces of poliovirus during routine testing of the wastewater at a north London sewage works has sparked an “urgent” investigation and provided an unwelcome reminder of a nasty disease that many might have assumed had been eradicated.

No cases of polio have yet been reported or confirmed in the UK but, if they were to be, they would be the first diagnosed in Britain for almost 40 years.

The US Centers for Disease Control and Prevention (CDC) describes polio or poliomyelitis as “a disabling and life-threatening disease caused by the poliovirus [that] spreads from person to person and can infect a person’s spinal cord, causing paralysis”.

It primarily affects children under five but, as the CDC points out, around 72 per cent of those infected will not experience any outward symptoms.

However, those that do will suffer a flu-like condition manifesting itself through sore throats, fever, fatigue, nausea, headaches and stomach pain, which can last for two to 10 days.

In much rarer instances, more like one per cent of cases, patients can suffer much more serious illness caused by the virus attacking the nerves in the spine and the base of the brain, which can cause meningitis or paralysis and lead to permanent disability.

In even rarer cases, polio can prove fatal should the disease target the muscles around the lungs and impede breathing.

The CDC warns that polio is “very contagious” and lives on in an infected person’s throat and intestines, causing it to be transmitted by contact with their faeces or through droplets when they cough or sneeze, with a patient contagious for up to two weeks after becoming infected.

There are two types of vaccine used to fight the poliovirus: the inactivated poliovirus vaccine, which is most common and administered by injection, and the oral poliovirus vaccine (OPV), given as a tablet.

The former has been the only kind given in the US and UK since 2000 and 2004 respectively – typically in childhood – but the OPV is still used elsewhere around the world.

What has caused such alarm at the UK Health Security Agency over the samples of the disease recovered from London Beckon Sewage Works is that they indicate the “likely” spread of a vaccine-derived poliovirus type 2 (VDPV2) between closely-linked people in the surrounding area.

VDPV2 is a rare, mutated form of the virus derived from the OPV, whose spread is holding back international efforts to eradicate polio once and for all.

The variant is typically found in under-immunised communities with poor sanitation and, given that the OPV has not been offered in Britain for 18 years, its appearance is thought to be caused by the infection of someone vaccinated abroad.

The traces were found at the plant in question between February and May 2022, which is a concern given that the facility serves four million Londoners and because, although such samples are found annually, these particular examples appear to be related to one another and share mutations, suggesting that the virus is evolving.

The onset of the coronavirus pandemic in 2020 has also disrupted the administration of polio vaccines to children, causing uptake to fall, meaning that some under-fives may be more vulnerable than usual, increasing the possibility of an outbreak in a country that has been considered polio-free since 2003.

Vaccines have done much to push back the threat from polio globally since the World Health Assembly adopted a resolution for its total eradication in 1988, driving down “wild” case numbers by 99 per cent from the 350,000 seen that year to just 175 in pre-Covid 2019 and meaning that 18 million more people can walk today than might otherwise have been able to without intervention.

An estimated 1.5 million lives are thought to have been saved as a result of the same initiative.

At the point of the resolution’s adoption, polio was classified as endemic in 125 countries but, in 2022, that only applies to two nations: Afghanistan and Pakistan.