Health authorities in Britain have declared a national incident after finding evidence of local spread of polio virus in London.
No cases of polio have been identified to date and the risk to the public is low. But health authorities urged anyone not fully vaccinated against the polio virus, especially young children, to seek vaccines immediately.
“The majority of the UK population will be protected from vaccination in childhood, but in some communities with low vaccination coverage, individuals may continue to be at risk,” said Dr. Vanessa Saliba, epidemiologist consultant to 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 per year.
According to Dr. Shahin Huseynov, technical officer for the World Health Organization’s vaccine that is preventable, officials identified the virus as routine monitoring of the country’s sewers between February and May. diseases and vaccination program in Europe.
Genetic analysis suggests the samples have a common origin, most likely a person 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 significance 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 additional samples and trying to identify the source of the virus. But the wastewater treatment plant that identified the samples covers about 4 million people, nearly half the city, making it challenging to locate 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 has been ingested by someone else. The virus thrives in the gut and shows up 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 is actually paralyzed,” 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 program.
In the UK, polio immunization is done with an injected inactivated polio virus, which cannot be excreted in the faeces. But some countries of the world rely on an oral polio vaccine that contains a live, attenuated version of the virus. Immunized people can briefly shed this virus in their feces, which can then end up in the sewer.
That’s what health officials believe happened in this case. The virus in the collected samples was, according to Dr. Huseynov derived from a type of oral polio vaccine used to contain outbreaks.
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.
The wild polio virus has been eradicated from every country in the world except Afghanistan and Pakistan. But vaccine-derived polio continues to cause minor outbreaks, especially in low-vaccination communities.
“Polio persists in some of the poorest parts of the world. Until it is eradicated globally, the risk of importation and spread in the UK and elsewhere remains,” said Nicholas Grassly, a vaccine epidemiologist at Imperial College London.
The analysis so far 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 issue here is whether it’s circulating continuously in the UK or whether it’s an immunodeficient person,” 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 need to find that immunodeficient person.”