All children aged one to nine and living in Greater London will be offered a polio vaccine after the virus was detected in sewage.
The virus, which can cause paralysis, has been found 116 times in London’s waste water since February.
The urgent immunisation campaign will see nearly a million children offered the vaccine – including those already up to date with their jabs.
Polio is seen as a disease of the past in the UK after the whole of Europe was declared polio-free in 2003.
However, what is happening now is slightly complicated as the samples detected are linked to a polio vaccine used in other countries.
Parts of the world still dealing with polio outbreaks use the oral polio vaccine – which is safe, but uses a live virus. This gives a huge amount of immunity, but has the potential to spread from person to person in areas where not a lot of people are protected.
This becomes a problem if it continues to spread, as the safe form of the virus used in the vaccine can mutate and evolve until it can once again lead to paralysis.
The UK Health Security Agency (UKHSA) says most of the samples detected are the safe vaccine form of polio, but “a few” have mutated enough to be dangerous.
The alarm was first raised in June after a series of tests at Beckton Sewage Works, which serves north and east London. More detailed analysis of the sewage system has since found the virus in:
The UKHSA says genetic analysis of the samples suggests that spread of the virus “has gone beyond a close network of a few individuals”.
A meeting of the government’s vaccine experts – the Joint Committee on Vaccination and Immunisation – recommended a rapid booster campaign for children aged one to nine.
The aim is two-fold. First to reduce the risk of any child catching the virus and being paralysed and also to raise immunity levels so the virus finds it harder to spread.
The majority of people with the infection have no symptoms but some feel as if they have the flu, with: