People are being urged to ensure their polio vaccines are up to date after an outbreak of the virus was detected in UK sewage samples.

A national incident has been declared by the UK Health Security Agency after traces of polio virus were found during a routine sewage check in London.

While it is normal for the virus to be picked up as isolated cases and not detected again, experts have raised the alarm after several genetically-linked viruses were found in samples between February and May.

Previously, the virus has been picked up when a person 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 in the recent samples has evolved in England and is now classified as a ‘vaccine-derived’ poliovirus type 2 (VDPV2).

VDPV is a strain of the weakened poliovirus, that was initially included in the oral polio vaccine, which has changed over time and behaves more like the “wild” or naturally-occurring virus.

This means it can be spread more easily to people who are unvaccinated and who come into contact with the faeces or coughs and sneezes of an infected person.

Dr Vanessa Saliba, a consultant epidemiologist at the UKHSA, said: "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 vaccination via NHS

While the UKSHA say risk is currently low, parents should ensure their children are fully immunised.

The polio vaccine is given on the NHS when a child is eight, 12 and 16 weeks old as part of the 6-in-1 vaccine. It is given again at three years and four months old as part of the 4-in-1 (DTaP/IPV) pre-school booster, and at 14 as part of the 3-in-1 (Td/IPV) teenage booster.

All of these vaccines need to have been given for a person to be fully vaccinated, though babies who have had two or three doses will have substantial protection.

Latest figures show that by the age of two in the UK, almost 95% of children have had the correct number of doses. However, this drops to just under 90% in London.

When it comes to the pre-school booster, just 71% of children in London have had it by the age of five.

If you are unsure you should contact your child's GP.