The UK health authorities issued a nationwide appeal on Wednesday for parents to ensure their children's vaccines are up to date after the polio virus was detected in sewage samples in London.
The UK Health Security Agency (UKHSA) said investigations are underway after several closely-related viruses were found in sewage samples taken between February and May.
The virus has continued to evolve and is now classified as a vaccine-derived poliovirus type 2 (VDPV2), which on rare occasions can cause serious illness, such as paralysis, in people who are not fully vaccinated.
The discovery is cause for concern as the last case of wild polio contracted in the UK was confirmed in 1984 and the UK was declared polio-free in 2003.
Vaccine-derived poliovirus is rare and the risk to the public overall is extremely low, said Dr Vanessa Saliba, Consultant Epidemiologist at UKHSA.
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, she said.
According to the UKHSA, 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.
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 UKHSA said that as part of routine surveillance, it is normal for one to three vaccine-like polioviruses to be detected each year in UK sewage samples but these have always been one-off findings that were not detected again.
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.
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.
The UK is considered by the World Health Organisation to be polio-free, with low-risk for polio transmission due to the high level of vaccine coverage across the population. However, vaccine coverage for childhood vaccines has decreased nationally and especially in parts of London over the past few years, so UKHSA is urging people to check they are up to date with their vaccines.
Wastewater surveillance is being expanded to assess the extent of transmission and identify local areas for targeted action. UKHSA said that healthcare professionals have been alerted to these findings so they can promptly investigate and report anyone presenting with symptoms that could be polio, such as paralysis.
The primary polio vaccine course is given to babies at two, three and four months. Three doses are needed to complete the primary course. In the UK it is given as part of the six-in-one vaccine.