After Poliovirus was found in London, the United Kingdom declared emergency

Health authorities in Britain have declared a national incident after finding evidence suggesting local spread of poliovirus in London.

No cases of polio have been identified so far, and the risk to the public is low. But health authorities urged anyone who is not fully immunized against poliovirus, especially young children, to seek vaccinations immediately.

“Most of the UK population will be protected against juvenile vaccination, but in some communities with low vaccination coverage, individuals may remain at risk,” said Drs. Vanessa Saliba, an advisory epidemiologist for the UK Health Security Agency.

The last case of polio in Britain 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 8,000 cases of paralysis reported each year.

Routine surveillance of sewage in the country catches poliovirus once or twice a year, but between February and May, officials identified the virus in several samples collected in London, according to Drs. Shahin Huseynov, Technical Officer for the Vaccine Prevention and Immunization Program of the World Health Organization in Europe.

Genetic analysis suggests that the samples have a common origin, probably an individual who traveled to the country around the New Year, Dr. Huseynov said. The last four collected samples appear to have evolved from this first introduction, probably in non-vaccinated children.

“The importance of this finding is that even in well-developed countries, the countries where the usual vaccination coverage is quite high, it is still important to ensure that all children have access to vaccines,” he said.

British officials are now collecting extra samples and trying to identify the source of the virus. But the wastewater treatment plant that identified the samples occupies about 4 million people, almost half of the city, making it challenging to identify the source.

Polio is most commonly spread by an infected person who does not wash their hands properly and then touches food or water that has been ingested by someone else. The virus flourishes in the gut and originates 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 one in 500 children who are actually lame,” he said. Dr. David Heymann, an expert in infectious diseases at the London School of Hygiene and Tropical Medicine who previously led the WHO’s polio eradication program.

In Britain, immunization for polio is carried out with an injected inactivated poliovirus, which can not be excreted by feces. But some countries around the world rely on an oral polio vaccine that contains a live, attenuated version of the virus. Immunized humans can briefly release this virus into their feces, which can then turn up in sewage.

That’s what health officials think has happened in this case. According to Dr. Huseynov.

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

Wild poliovirus has been eliminated from all countries in the world except Afghanistan and Pakistan. But vaccine-derived polio continues to cause small outbreaks, especially in communities with low vaccination coverage.

“Polio remains in some of the poorest parts of the world. Until it is eliminated worldwide, the risk of importation and spread in the UK and elsewhere will continue,” said Nicholas Grassly, a vaccine epidemiologist at Imperial College London.

The analysis suggests so far transmission from the community, probably among young children. A less likely possibility is that a single immunocompromised individual has lost the virus for months.

“The big problem here is whether it is continuously circulating in the UK or whether it is an immunodeficient person,” said Drs. Walter Orenstein, associate director of the Emory Vaccine Center and former director of the United States Immunization Program.

If the latter, Orenstein said, “they need to find that immunodeficient person.”

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