Polio has been discovered in London sparking a flurry of activity by health officials and a warning for all people to get their vaccines checked.
Health bosses said the risk to the public was "extremely low" but the UK Heath Security Agency (UKHSA) said it was urgently investigating the discovery in north-east London.
What is polio?
Polio is a contagious virus that can be transmitted through coughs and sneezes, but also through food, water or objects that have been in contact with the faeces of someone infected with it.It can live in an infected person's throat for weeks without them suffering any symptoms.
Around 70% of people who have polio have no symptoms.
The last confirmed case of polio in the UK was in 1984 and it was declared eradicated in 2003.
In most cases it appears like the flu with mild symptoms, but in rare instances - more than one in 100 - it can affect the nerves in your spine and brain and cause paralysis.
This paralysis is usually in the legs and normal lasts a few weeks or months.
But it can be life-threatening if the paralysis affects the muscles in your neck that help with breathing.
For those who do suffer some form of paralysis, around 2 to 5% of children die and 15 to 30% of adults die.
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US President Franklin Roosevelt suffered from what is believed to have been polio in 1921, which left him needing to use a wheelchair for the rest of his life.
He founded a research centre for polio in 1938 which led to the development of the first vaccines against the virus.
The vaccines have effectively eradicated "wild polio" from the planet, and between 2018 and 2022 it was only reported in Afghanistan and Pakistan.
ITV News' Science Editor Deborah Cohen explains what the discovery of polio in sewage in North and East London means.
What has been discovered in the UK?
The UKHSA, working with the Medicines and Healthcare products Regulatory Agency (MHRA), has found polio in sewage samples collected from the London Beckton Sewage Treatment Works, which serves around four million people in north and east London.While it is normal for the virus to be picked up as isolated cases and not detected again, experts have raised the alarm after several genetically-linked viruses were found in samples between February and May.The strain of polio is not the natural "wild" strain, but instead, it is a weaker form of the virus that is included in the oral vaccination.
It is believed someone who received the oral vaccination brought the weakened case to the UK from abroad and then shed traces of it in their faeces.
However, this weakened polio has since evolved into ‘vaccine-derived’ poliovirus type 2 (VDPV2).
VDPV is a strain of the weakened poliovirus, that was initially included in the oral polio vaccine, which has changed over time and behaves more like the “wild” or naturally-occurring virus.
This means it can spread more easily to people who are unvaccinated and who come into contact with the faeces or coughs and sneezes of an infected person.The UKHSA is working on the theory that a person vaccinated abroad with the polio vaccine – possibly in Afghanistan, Pakistan or Nigeria – entered the UK early in 2022 and was shedding the virus.That person has now passed it onto other, closely linked individuals in north-east London, who in turn are shedding the virus into their faeces.Is there anything to worry about?
For the vast majority of the UK there is nothing to worry about, all children in the UK have been vaccinated against polio for decades.
The NHS has urged Londoners to double-check they have had the vaccine and if they may need a top-up.
Jane Clegg, Chief nurse for the NHS in London said: "The majority of Londoners are fully protected against polio and won't need to take any further action, but the NHS will begin reaching out to parents of children aged under five in London who are not up to date with their polio vaccinations to invite them to get protected.
"Meanwhile, parents can also check their child's vaccination status in their Red Book and people should contact their GP Practice to book a vaccination should they or their child not be fully up to date."