Health authorities in the UK have declared a national incident after finding evidence pointing to local spread of poliovirus in London.
No cases of polio have been identified to date and the risk to the public is low. However, health officials have urged anyone who is not fully immunized against the poliovirus, especially young children, to seek vaccines immediately.
“Most of the UK population will be protected from vaccination during childhood, but in some communities with low immunization coverage, individuals may remain at risk,” said Dr. Vanessa Saliba, Advisory Epidemiologist at the UK Health Authority.
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 widespread in the UK, with up to 8,000 cases of paralysis being reported each year.
Routine monitoring of the country’s sewage picks up the poliovirus once or twice a year, but officials identified the virus in several samples collected in London between February and May, according to Dr. Shahin Huseynov, Technical Officer for Vaccination Prevention, World Health Organization Diseases and immunization programs in Europe.
Genetic analyzes suggest the samples share a common origin, most likely an individual who traveled to the country around the New Year, said Dr. Huseynov. The last four samples collected appear to have evolved from this initial introduction, probably in unvaccinated children.
“The importance of this finding is that even in well-developed countries, the countries where the usual immunization coverage is quite high, it’s still important to make sure all children have access to vaccines,” he said.
British officials are now collecting additional samples and trying to identify the source of the virus. But the sewage treatment plant that identified the samples covers about 4 million people, nearly half the city, making it difficult to pinpoint the source.
Polio is most commonly transmitted by an infected person who does not wash their hands properly and then touches food or water ingested by another person. The virus thrives in the gut and appears 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, only about 1 in 500 children is actually paralyzed,” 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 the UK, immunization against polio is given with an injected inactivated poliovirus that cannot be shed through the faeces. However, some countries around the world rely on an oral polio vaccine that contains a live, weakened version of the virus. Vaccinated people can shed this virus briefly in their faeces, which can then show up in the sewage.
That’s what health authorities believe is happening in this case. According to Dr. Huseynov of a type of oral polio vaccine used to contain outbreaks.
In recent months, this 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 in every country in the world except Afghanistan and Pakistan. But vaccine-induced polio continues to cause small outbreaks, particularly in communities with low immunization coverage.
“Polio is in some of the poorest parts of the world. Until it is eradicated globally, the risk of introduction and spread in the UK and elsewhere remains,” said Nicholas Grassly, vaccine epidemiologist at Imperial College London.
Analysis to date suggests community transmission, most likely among young children. A less likely possibility is that a single immunocompromised person has been shedding the virus for months.
“The big question here is whether it’s constantly circulating in the UK or whether it’s from an immunocompromised individual,” said Dr. Walter Orenstein, associate director of the Emory Vaccine Center and former director of the United States Immunization Program.
If it’s the latter, Orenstein said, “they have to find this immunocompromised person.”