"On rare occasions it can cause paralysis in people," health officials warned
Health officials are "urgently investigating" after polio was found in sewage in North and East London.
The UK Health Security Agency said poliovirus, which can cause paralysis in unvaccinated people, was discovered at the London Beckton Sewage Treatment Works.
Health bosses said the risk to the public was "extremely low" but urged people to check their vaccination status.
"Vaccine-derived poliovirus has the potential to spread, particularly in communities where vaccine uptake is lower," said Dr Vanessa Saliba, from the UK Health Security Agency.
"On rare occasions it can cause paralysis in people who are not fully vaccinated so if you or your child are not up to date with your polio vaccinations it’s important you contact your GP to catch up or if unsure check your red book.
"Most of the UK population will be protected from vaccination in childhood, but in some communities with low vaccine coverage, individuals may remain at risk," she explained.
ITV News' Science Editor Deborah Cohen explains what the discovery of polio in sewage in North and East London means.
What is vaccine-derived poliovirus?
Despite the eradication of wild poliovirus, in some countries vaccines are given with oral drops.
They contain a weakened form of the virus which develops immunity and builds antibodies.
The virus in the oral vaccine is also excreted which means if a population is under-immunised it can continue to circulate.
On very rare occasions, the vaccine-virus can mutate into a form that can paralyse, known as a circulating vaccine-derived poliovirus, which behaves more like wild polio.
The virus is spread by poor hand hygiene, in particular when an infected person does not wash their hands after using the toilet and then touching food or water consumed by others and, less commonly, through coughing and sneezing.
'Majority of Londoners are fully protected'
"The majority of Londoners are fully protected against polio and won’t need to take any further action, but the NHS will begin reaching out to parents of children aged under five 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.
Health authorities said it was "normal" for 1 to 3 'vaccine-like' polioviruses to be detected each year in UK sewage samples.
Previous cases occurred when an individual vaccinated overseas with the oral polio vaccine returned to the UK and "shed" traces of the vaccine-like poliovirus in their faeces.
The current investigation began when several closely related viruses were found in sewage samples taken between February and May.
The virus continued to evolve and is now officially classified as a 'vaccine-derived' poliovirus type 2 (VDPV2).
"The detection of a VDPV2 suggests it is likely there has been some spread between closely linked individuals in North-East London and that they are now shedding the type 2 poliovirus strain in their faeces," the UK Health Security Agency said.
"The virus has only been detected in sewage samples and no associated cases of paralysis have been reported but investigations will aim to establish if any community transmission is occurring," the agency added.
Oral polio vaccine has not been used in the UK since 2004.
The last case of wild polio contracted in the UK was confirmed in 1984 and the UK was declared polio-free in 2003.
What are the symptoms of polio?
Most people with polio won't have any symptoms and will fight off the infection without even realising they were infected.
A small number of people will experience a flu-like illness 3 to 21 days after they're infected.
Symptoms can include:
A high temperature (fever) of 38C (100.4F) or above
A sore throat
Abdominal (tummy) pain
Feeling and being sick
These symptoms will usually pass within about a week without any medical intervention.
In a small number of cases, between 1 in 100 to 1 in 1000 infections, the polio virus attacks the nerves in the spine and base of the brain.
This can cause paralysis, usually in the legs, that develops over hours or days. If the breathing muscles are affected, it can be life threatening.
When should my child be vaccinated?
The vaccine is given as part of a combined jab to babies, toddlers and teenagers as part of the NHS routine childhood vaccination schedule.
It is given when your child is:
8, 12 and 16 weeks old as part of the 6-in-1 vaccine
3 years and 4 months old as part of the 4-in-1 (DTaP/IPV) pre-school booster
14 years old as part of the 3-in-1 (Td/IPV) teenage booster
You need to have all of these vaccinations to be fully vaccinated against polio.
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