A sewage site in the Watford area is to be tested for traces of polio after samples of the virus were discovered in London.

The UK Health Security Agency (UKHSA) says it is expanding its surveillance to 20 other places to see whether the virus is spreading to other areas.

The sampling of the sewage treatment works serving people living in and around Watford is on a “precautionary basis” and due to its proximity to London.

Dr Vanessa Saliba, consultant epidemiologist at UKHSA, said: “No cases of polio have been reported and for the majority of the population, who are fully vaccinated, the risk is low.

“We are now expanding the sewage sampling nationally to areas that are at highest risk of new poliovirus importations and areas most likely to see spread of poliovirus from London.

“We are in touch with public health colleagues in these areas and will work closely with local areas as the need arises.”

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The full list of areas where the sampling will be rolled out are at sewage treatment works covering parts of Watford, Luton, Birmingham, Manchester, Liverpool, Newcastle, Sheffield, Leeds, Leicester, Nottingham, Bristol, Brighton, Blackburn with Darwen, Bradford, Bury, Castle Point, North Tyneside, Gateshead, Preston, and Salford, in addition to work already ongoing in Glasgow and Beckton in London.

The UKHSA says its sewage sampling strategy will continue to be reviewed and adapted as new evidence emerges.

Nationally the overall risk of paralytic polio is considered “low” because most people are protected from this by vaccination.

The alarm was first sounded in June after poliovirus was discovered in Beckton sewage works in east London.

Parents across the country are being asked to check their children are fully vaccinated for their age.

Dr Saliba said: “Following advice from the Joint Committee on Vaccination and Immunisation (JCVI), all children aged 1 to 9 in London need to have a dose of polio vaccine now – whether it’s an extra booster dose or just to catch up.

“It will ensure a high level of protection from paralysis. This may also help stop the virus spreading further.”

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