Polio in the United Kingdom: Should the rest of the world be concerned?

Traces of the virus were detected in an East London sewage plant, prompting a "national incident" to be declared on Friday.

Infectious diseases expert at the Australian National University, Associate Professor Sanjaya Senanayake, said that while the detection was cause for concern, it is unlikely that Australia will see a resurgence of the mostly-eradicated virus.

"If you look at the UK Health Security Agencies announcements, probably every year they get one to three cases or situations where they find polio in the sewage every year," Professor Senanayake said.

"But they are unrelated genetically. So that does happen. But what they're seeing now in the UK is between February and June, a large number of cases in an area which is a population of about 4 million people where the polio virus they've identified is genetically related."

So how did polio seemingly return to the United Kingdom overnight, after 40 years without an active case?

Since the worldwide vaccination program began, cases of wild poliovirus have declined by about 99.9 per cent.

The last natural infection of polio was recorded in the United Kingdom in 1984, and since 2003 the UK has been declared officially polio-free.

Though the virus is now considered eradicated in most countries, it continues to circulate in Afghanistan and Pakistan. There, it is still considered endemic.

But outside these endemic nations, small outbreaks or detections of the virus still do happen. When this happens, health authorities look to see which form of the virus is being found in greater abundance.

There are three polio strains, known as P1, P2, and P3. Each differs in its severity.

Type three was only officially eradicated in 2019 though the last wild case was recorded in northern Nigeria sometime around 2012.

The second type was stamped out in 2015, and the first type continues to circulate in very low numbers. It is this type that is most commonly found in low-vaccinated places, including in Pakistan and Afghanistan.

But then there's a whole other separate strain of polio, that of vaccine-related polio.

There are two types of vaccines for polio. One is an injection, and one is administered orally.

The injection contains an inactive strain of the virus, but the oral form includes a live virus that is then ingested and sits in the gut.

It's not a form that is common across much of the world anymore, but historically, the oral vaccine was given to children usually on top of a sugar cube or with syrup.

"The good thing about the oral polio vaccine is that even though it's alive, it's excreted in the poo and gets into the sewage," explained Professor Senanayake.

"And by doing that, in countries with low [vaccine] coverage and lots of polio, particularly third world countries, it means that if people don't wash their hands properly or eat contaminated food, they can actually get the vaccine-derived poliovirus from the sewage and they can also get vaccinated or immunised."

There is no risk of contracting the live poliovirus from the injected form of the vaccine, but there is very low risk with the oral form.

That live form of the virus also has a chance of mutating and becoming more aggressive.

For this reason, many parts of the world, including the United Kingdom and Australia, have moved away from this form of vaccination.

The United Kingdom has not used the oral vaccine since 2004.

"About one in 3 million people can actually get neurological symptoms from the oral vaccine itself," Professor Senanayake said.

Before vaccination programs began, there was a one-in-200 person chance of catching polio in the wild.

"And when polio was running around the world, one-in-200 meant large numbers, particularly of children who were getting paralysis," Professor Senanayake said.

"And of course, the most famous one in recent history is FDR, Franklin Delano Roosevelt, the US president who got polio and was wheelchair-bound. But so many were affected.

"You and I haven't lived through polio, but if you talk to people who have, it was pretty scary, you know, kids getting paralysed and to hear about the iron lungs for polio as well for people who respiratory function was affected. It was a horrible time."

The type of polio that has been detected in East London is believed to have been derived from the oral vaccination.

Because this form of vaccination is not used in the United Kingdom, Professor Senanayake believes it's likely arrived with people who have come from overseas.

"They've come to the UK and it's been circulating there, but it hasn't caused significant clinical disease wherein a hospital, someone has said, 'Oh, my God, there's a paralysis case, let's look for polio'," he said.

"But the concern is that if the circulation continues, eventually it will lead to that."

We tend to associate the poliovirus with images of crippled children and iron lungs, but for a long time, this has not been the world's experience with the virus.

In fact, about 75 per cent of us will have had polio at one point in our lives, but we will not have known.

"It can be an asymptomatic disease," Professor Senanayake said.

"The vast majority of the 25 per cent who do get symptoms, it's like a flu-like illness basically for most people."

Health authorities all over the world will be closely watching what happens in the United Kingdom, but it's very unlikely that we'll see a worldwide return to those pre-vaccination days.

It is unlikely poliovirus will become endemic again.

"They'll [UK health department] will be looking to see how extensive it is, so they've got to ramp up their surveillance," Professor Senanayake said.

Our populations are returning to 'normal' following years of long lockdowns due to the pandemic.

During these lockdowns, many virus transmissions were circumvented, which has led our societies to be more vulnerable to catching things now that we're re-opening.

It's why we're seeing spikes in influenza cases all over the world.

"So I guess even before COVID started in 2020, they did see an increase in cases. So there'd been maybe a few dozen cases in 2016 and then maybe a few hundred in 2019, 2020 so there had been an increase," Professor Senanayake said.

"But I think with COVID, the concern is that we focused so much on COVID, particularly in developing countries, that other vaccination programs may have gotten neglected just due to resource issues."