After the discovery of the polio virus in London, the United Kingdom declares a state of emergency

Britain’s health authorities have declared a national incident after finding evidence of the polio virus spreading locally in London.

No cases of polio have yet been identified, and the risks to the public are low. But health authorities have urged anyone who has not been fully immunized against the polio virus, especially young children, to seek vaccinations immediately.

“Most of the UK population will be protected from childhood vaccination, but in some communities with low vaccine coverage, individuals may still be at risk,” said Dr Vanessa Saliba, a consultant epidemiologist with the UK Health Security Agency.

Britain’s last case of polio was in 1984, and the country was declared polio-free in 2003. Before the introduction of the polio vaccine, epidemics were common in Britain, with up to 8000 cases of polio reported each year.

Routine monitoring of the country’s wastewater detects polio virus once or twice a year, but between February and May, officials identified the virus in several samples collected in London, according to Dr. Disease and Immunization Program in Europe.

Dr. Huseynov said genetic analysis indicates the samples have a common origin, most likely an individual who traveled to the country in the new year. The last four samples collected appeared to have developed from this initial introduction, most likely in unvaccinated children.

“The significance of this finding is that even in developed countries, and countries where usual immunization coverage is very high, it is still important to ensure that all children get vaccinations,” he said.

British officials are now collecting additional samples and trying to determine the source of the virus. But the sewage treatment plant that identified the samples covers about 4 million people, nearly half of the city, making it difficult to pinpoint the source.

Polio is most often spread by an infected person who does not wash their hands properly and then touches food or water that someone else has eaten. The virus spreads in the gut and is excreted in the feces of infected people. In up to 1 percent of patients, the virus can infect the spine and cause paralysis.

“Most of the disease is asymptomatic, it’s only about 1 in 500 children who are actually paralysed,” he said. Dr David Heymann, an infectious disease expert at the London School of Hygiene and Tropical Medicine who previously led the WHO’s polio eradication programme.

In Britain, polio vaccination is carried out with an injection of inactivated polio virus, which cannot be eliminated through the faeces. But some countries in the world rely on the oral polio vaccine, which contains a live, weakened version of the virus. Immunized people can briefly shed this virus in their faeces, which can then show up in sewage.

That’s what health officials believe happened in this case. The virus came in samples collected from a type of oral polio vaccine used to contain outbreaks, according to Dr. Huseynov.

In recent months, he said, this type of vaccine has only been used in Afghanistan, Pakistan and some countries in the Middle East and Africa.

Wild poliovirus has been eliminated from every country in the world, except for Afghanistan and Pakistan. But vaccine-derived polio still causes small outbreaks, particularly in communities with low vaccination coverage.

“Polio persists in some of the world’s poorest regions. Until it is eradicated worldwide, the risk of importing and spreading to the UK and elsewhere will persist, said Nicholas Grassley, a vaccine epidemiologist at Imperial College London.

The analysis to date indicates community transmission, most likely among young children. The least likely possibility is that the immunocompromised individual has shed the virus for months.

“The big issue here is whether it is circulating consistently in the UK or whether it is someone who is immunocompromised,” said Dr Walter Orenstein, associate director of the Emory Vaccine Center and former director of the US Immunization Program.

If it’s the latter, Orenstein said, “they need to find that immunocompromised person.”