While an investigation has been opened, no cases so far have been reported. The United Kingdom (UK) has been polio-free since 2003, according to the UK Health Security Agency (HSA).

Besides investigating whether there is a community transmission in some communities in London, the authorities have also expanded wastewater surveillance to assess the extent of transmission and identify local areas for targeted action, said HSA in a statement.

Here we explain the significance of the discovery of several polio virus samples, the status of disease across the world, how British authorities plan to investigate possible transmission, and how it could spread in polio-free regions.

What we know of polio virus in London

Of few "one-off" polio virus samples detected every year, the HSA said, "These previous detections occurred when an individual vaccinated overseas with the live oral polio vaccine (OPV) returned or travelled to the UK and briefly 'shed' traces of the vaccine-like poliovirus in their faeces."

However, the virus detected this year has evolved and has been classified as 'vaccine-derived' poliovirus type 2 (VDPV2). The HSA said it can on rare occasions cause serious illness, such as paralysis, in people who are not fully vaccinated.

The HSA highlighted it's likely there is community spread to some extent in London.?

It said, "The detection of a VDPV2 suggests it is likely there has been some spread between closely-linked individuals in North and East London and that they are now shedding the type 2 poliovirus strain in their faeces."

Dr Vanessa Saliba, Consultant Epidemiologist at HSA, said while the public health risk is "extremely low", the vaccine-derived polio virus has the most potential to spread.

She said, "Vaccine-derived poliovirus has the potential to spread, particularly in communities where vaccine uptake is lower. On rare occasions it can cause paralysis in people who are not fully vaccinated."?

How polio spreads, its global status

There are three types of wild polio viruses, of which two have been eradicated and the remaining third is endemic in only Pakistan and Afghanistan, according to the World Health Organization (WHO).

Polio virus can spread via droplets of infected persons or with contact with water or faecal matter contaminated with the virus, according to Healthline.

It adds, "People living in areas with limited access to running water or flush toilets often contract polio from drinking water contaminated by infected human waste. Objects like toys that have come near infected feces can also transmit the virus."

Poliovirus and vaccination

The virus has been eradicated from the world, except for Afghanistan and Pakistan, through large-scale vaccination programs.?

There are broadly two types of polio virus vaccines — injected inactivated virus (IPV) vaccine and oral polio vaccine (OPV). The UK uses IPV vaccines which carry inactivated virus.

IPV vaccines are safe and effective in protecting the immunised person from paralysis, but it is less effective at inducing local immunity in the gut, so vaccinated people can still become infected and shed infectious virus, even though they may not show symptoms themselves, said immunologist Zania Stamataki in an article in The Conversation.

She added, "IPV offers excellent protection for the individual, but is not enough to control an epidemic in poor sanitation conditions."

For such conditions, OPV vaccines are "ideal", according to Stamataki. But there are complications with these vaccines, which are not used in the United States and UK.

She wrote, "The downside of using OPV is that the weakened virus can mutate, and in rare cases, it can revert to paralysis-causing variants."

She added that this can lead to vacine-derived poliovirus in under-immunised countries. This is exactly what has happened in London, which has a polio vaccination rate lower than the rest of UK, according to BBC.

The BBC reported, "Take-up of the first three [polio vaccine] doses is about 86 per cent in London, well below target levels, with the rest of the UK over 92 per cent."

Sky News reported that it's likely the virus was shed by someone who was recently vaccinated in a country where it has not yet been eradicated, such as Pakistan, Afghanistan, or Nigeria.

Steps being taken in London

Besides expanding wastewater surveillance and checking for the wider community spread, the authorities would also reach out to parents to complete their children's vaccination.

"NHS will begin reaching out to parents of children aged under 5 in London who are not up to date with their Polio vaccinations to invite them to get protected," said Jane Clegg, Chief nurse for the NHS in London.

The Sky News reported that the alert level would be escalated to the next level if more virus samples are found, which will see targeted interventions such as including small batch vaccinations and the collection of stool samples from areas where the polio virus has been found.

Professor Adam Finn, a member of the Joint Committee on Vaccination and Immunisation, told Sky News that the evidence "strongly suggests" there is transmission between people.?

He said, "And when we see importations that are like a one off, if you like, somebody who's recently been vaccinated, that usually just pops up for a short time and then disappears, whereas we're seeing genetically related viruses persistently over a period of months now. And that strongly suggests that there's transmission going on between people within the UK and in this area of London in particular."

Wastewater surveillance's significance

This is yet another example where wastewater surveillance has found healthcare applications. Not just polio, but wastewater has also been used across the world to track coronavirus spread and the spread of strains in particular areas.

While the most common way to shed the coronavirus is through nose or mouth, but people also shed the virus in faecal matter.

"Regularly analysing wastewater from sewage treatment plants allows scientists to measure when those levels are rising or falling and what variants are present about four to five days before people start testing positive," notes a piece in The Washington Post.

The idea behind wastewater analysis is to produce regional, community-level anonymous data for public health purposes that gives an idea about the prevalence of a disease in a wider population.