Parents have been warned to make sure their children are up to date on all their vaccinations as polio is thought to be spreading in Britain for the first time in decades.
It comes after the virus which causes polio, a disease common in the UK in the 1950s but eradicated by 2003, was detected in a number of sewage samples in Beckton, East London.
While the risk is low, parents have still been reminded to ensure their children are fully protected.
The UK Health Security Agency (UKHSA) has now declared a national incident and alerted the World Health Organisation, which could strip Britain of its polio-free status.
After the discovery, Jane Clegg, chief nurse for the NHS in London, said: “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 vaccinations to invite them to get protected.
“Meanwhile parents can also check their child’s vaccination status in their Red Book and people should contact their GP Practice to book a vaccination should they or their child not be fully up-to-date.”
The UKHSA says the sewage samples could have been imported by someone who was vaccinated overseas with a live form of the virus and then briefly "shed" traces of the vaccine-like poliovirus in their faeces.
It's important that vaccines are given on time for the best protection, but if a child misses a vaccine, parents should contact their GP to sort a catch-up appointment.
The polio vaccine is given when a child is eight, 12 and 16 weeks old as part of the 6-in-1 vaccine, at three years and four months old as part of the 4-in-1 (DTaP/IPV) pre-school booster, and at 14 years old as part of the 3-in-1 (Td/IPV) teenage booster.
All these vaccinations are needed to be fully vaccinated against polio.
The last case of wild polio contracted in the UK was confirmed in 1984, with the country declared polio-free in 2003.
Dr Vanessa Saliba, consultant epidemiologist at UKHSA, said: “Vaccine-derived poliovirus is rare and the risk to the public overall is extremely low.
“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 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.
“We are urgently investigating to better understand the extent of this transmission and the NHS has been asked to swiftly report any suspected cases to the UKHSA though no cases have been reported or confirmed so far.”
Polio is a contagious virus that can be transmitted through coughs and sneezes.
However, it can also be transmitted through food, water or objects that have been in contact with the faeces of someone infected with it, and it can live in an infected person's throat and intestines for weeks.
You can have a polio vaccination at any point if you've never had one before, even if you're not travelling to a country with a risk of getting polio, the NHS says.
Most people who get polio don't have symptoms. However, some people get mild, flu-like symptoms, such as: a high temperature, extreme tiredness (fatigue), headaches, being sick (vomiting), a stiff neck or muscle pain.
These symptoms usually last up to 10 days.
In rare cases, polio can cause difficulty using your muscles - or paralysis - usually in the legs. This can happen over hours or days.
It's not usually permanent and movement will slowly come back over the next few weeks or months, but it can be life threatening if the paralysis affects the muscles used for breathing. In one in 200 cases it causes irreversible paralysis.
The NHS urges anyone who has polio symptoms and who has travelled to a country where polio is found to call 111.
Polio is now mainly found in just two countries - Afghanistan and Pakistan.
The attenuated oral vaccine offered abroad is known to be able to trigger an infection which then sees the virus mutate and spread, but only in one in a million cases.
