UK health authorities will offer children under 10 living in London an additional dose of polio vaccine amid rising concern about a comeback of the disease.

While nobody has tested positive for infection with the type of polio virus that can cause paralysis, authorities have nonetheless found it in several London sewage samples recently. This suggests “there is some level of virus transmission” in parts of London that “has gone beyond a close network of a few individuals,” the UK Health Security Agency said in a statement.

The UK’s vaccine expert advisory committee recommended offering the extra polio booster to kids after considering London’s polio vaccination coverage, which is lagging behind World Health Organization targets.

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Of the 116 polio viruses isolated from London sewage samples in recent months, the vast majority are relatively harmless versions classified as “vaccine-like” virus. People can spread this version through their feces after receiving a live-attenuated oral version of polio vaccine that’s no longer offered in the UK.

If this version of the virus undergoes enough mutations, however, it sometimes becomes a more dangerous strain that can cause paralysis on rare occasions. On several occasions in London since February, this more dangerous strain, known as “vaccine-derived” polio virus, has been detected in sewage samples.

Normally the surveillance would only pick up one-off traces of the vaccine-like virus a year and the consistent detection in recent months merits further intervention, health officials said.

The goal of the booster program is to strengthen protection against polio in London and to slow down transmission, officials said. Nationally the overall risk of paralytic polio is considered low because most people are protected from this by vaccination, the UKHSA said. Meanwhile, authorities are also expanding sewage surveillance in London and the rest of the country to determine if the virus has spread elsewhere.

People can often transmit the polio virus without showing any symptoms, although some will experience a flu-like illness with fever, a sore throat and headache. Often these symptoms disappear without intervention but in some cases the virus attacks the nerves in the spine and base of the brain which can lead to paralysis. The virus can be life-threatening if the breathing muscles are affected.

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The UK’s last case of wild polio was in 1984 and the country was declared polio-free in 2003.

The US reported its first polio case in nearly a decade in July, from an unvaccinated 20-year-old man living in the New York State. Another case was reported in March in Israel as an unvaccinated girl developed paralysis and subsequently tested positive for polio.

The UK health authorities said they are working with peers in the US, Israel and the WHO to probe links between what’s detected in London and the cases reported in those two countries.

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