U.K. health authorities have said they are “urgently” investigating a rare poliovirus discovery in sewage samples in London.Picture Alliance | Getty ImagesU.K. health authorities have said they are urgently investigating a rare polio virus discovery in sewage samples in London, potentially putting Britain’s polio-free status at risk for the first time in almost two decades.A number of waste samples from the Beckton sewage treatment works in Newham, east London tested positive for vaccine-derived polio virus between February and May, the U.K. Health Security Agency said Wednesday.The virus has since continued to evolve and is now classified as a “vaccine-derived” polio virus type 2, the UKHSA said, adding that it is looking to establish if any community transmission is occurring.The agency has declared a national incident and informed the World Health Organization of the situation.”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,” Dr. Vanessa Saliba, consultant epidemiologist at UKHSA, said Wednesday.Polio is a rare virus that can occasionally cause serious illness, such as paralysis, in people who are not fully vaccinated. The disease was previously common in the U.K. in the 1950s, but the country was declared polio-free in 2003.The UKHSA said the risk to the general public is extremely low, but urged parents to ensure their children have been fully immunized against the disease. It is common practice in the U.K. for children to receive an inactivated polio vaccine as part of their routine vaccination program; with three shots given before the age of one and another shot given at ages three and 14.”Most of the U.K. population will be protected from vaccination in childhood, but in some communities with low vaccine coverage, individuals may remain at risk,” Saliba said.Each year, it is usual for one to three “vaccine-like” polio viruses to be detected in Britain’s sewage system. Such detections have always been one-off findings, and have previously occurred when an individual vaccinated overseas with the live oral polio vaccine returned or traveled to the U.K. and briefly “shed” traces of the vaccine-like polio virus in their feces.However, this marks the first time a cluster of genetically-linked samples has been identified repeatedly over several months.Vaccination statusScientists say that this suggests there has been some community spread between closely-linked individuals in north and east London.So far, the virus has only been detected in sewage samples, and no associated cases of paralysis have been reported, according to the UKHSA.While vaccination against polio is commonplace in the U.K., immunization rates vary across the country, with communities with lower uptake at greater risk.Vaccine coverage for childhood vaccines, in particular, has waned nationally and especially in parts of London over recent years.The U.K.’s National Health Service said parents should contact their doctor’s surgery to check their child’s vaccines are up to date.”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 5 in London who are not up to date with their Polio vaccinations to invite them to get protected,” Jane Clegg, chief nurse for the NHS in London, said.”Meanwhile, parents can also check their child’s vaccination status in their Red Book and people should contact their GP surgery to book a vaccination, should they or their child not be fully up to date,” she added.