Health officials have declared a national incident as they investigate the scale of a feared polio outbreak following the discovery of the virus in wastewater.
The virus, which can cause paralysis in rare cases, was identified in sewage samples taken from London between February and May 2022, according to the UK Health Security Agency.
UKHSA said it was “likely” there has been spread of a vaccine-derived poliovirus type 2 (VDPV2) between closely linked people in northeast London, with infected individuals believed to be shedding the virus in their faeces. So far, no suspected cases have been reported or confirmed.
The agency is now “urgently” investigating whether wider transmission of the infection, which can spread without symptoms in low-vaccinated communities, is occurring in the capital. If confirmed, this would be the first outbreak of polio in the UK in nearly 40 years.
Dr Vanessa Saliba, a consultant epidemiologist at UKHSA, said: “Most of the UK population will be protected from vaccination in childhood, but in some communities with low vaccine coverage, individuals may remain at risk.
“We are urgently investigating to better understand the extent of this transmission and the NHS has been asked to swiftly report any suspected cases to the UKHSA, though no cases have been reported or confirmed so far.”
The rise of VDPV2s throughout the globe is threatening attempts to eradicate polio. They are a rare, mutated version of the virus – typically found in under-immunised communities with poor sanitation – which derive from the live oral polio vaccine (OPV).
People vaccinated with OPV, which has not been used in Britain since 2004, can briefly shed traces of the vaccine-like poliovirus in their faeces. Samples of this kind are detected up to three times a year in UK sewage, likely from peopled innoculated overseas, and are considered normal, the UKHSA said.
However, the viral samples found earlier this year at London Beckon Sewage Works, which covers 4 million people across the northeast of the capital, appear to be related and share mutations. This suggests the virus is spreading and evolving.
It’s likely the virus was introduced to the UK by a person vaccinated with the OPV in an overseas country. From there, it may have then spread within a single family.
“These findings suggest that there may be localised spread of poliovirus, most likely within individuals that are not up to date with polio immunisations,” said Dr Kathleen O’Reilly, an associate professor in statistics for infectious disease.
So far, there have been no associated cases of polio-related paralysis, which occurs in up to 1 per cent of infections. Most people who contract the virus will not develop any visible symptoms. UKHSA said the risk to the public is “extremely low”.
Jane Clegg, chief nurse for the NHS in London, said health officials will begin reaching out to parents of children aged under five who are not up to date with their polio vaccinations.
“The majority of Londoners are fully protected against polio and won’t need to take any further action,” she added.
The last case of wild polio contracted in the UK was confirmed in 1984 and the UK was declared free of the virus in 2003. Afghanistan and Pakistan are the only two countries in the world where the infection is still classified as endemic.
In response to the outbreak, wastewater surveillance is being expanded to six other areas in the north of London to determine the extent of transmission and identify local areas for targeted action.
The UKHSA said that if other outbreaks were found it might roll out a mass vaccination programme to make sure communities were protected.
Healthcare professionals have been told to investigate and report anyone presenting with symptoms that could be polio. While the majority of patients won’t display symptoms, one in four will develop a flu-like illness three to 21 days after infection. Symptoms can include fever, nausea and stomach pain.
In rare cases, poliovirus attacks the nerves in the spine and base of the brain of an infected individual. This can cause paralysis, usually in the legs, that develops over hours or days. If the breathing muscles are affected, it can be life threatening. Up to 10 per cent of people who develop paralysis from polio die.
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Live OPV is used in poorer parts of the world to respond to polio outbreaks. This vaccine generates gut immunity and for several weeks after receiving it people can shed the vaccine-virus in their faeces.
These vaccine-viruses can then spread in under-vaccinated communities from person to person through poor hand hygiene and water and food contamination. Coughing and sneezing, though less common, is another route of transmission.
As it spreads, as appears to have been the case in Britain over a number of months, the virus can mutate into a vaccine-derived version of poliovirus, which behaves more like the naturally occurring “wild” type. VDPV2 is the most prevalent kind, with 959 cases detected worldwide in 2020.
UK coverage for all childhood vaccinations, which covers diphtheria, tetanus, polio, pertussis, hib and hepatitis B, plateaued between 2011-12 and 2013-14, and has since declined, according to analysis from the Nuffield Trust.
This decline has been particularly acute in parts of London in recent years, UKHSA said on Wednesday.
In 2020-21, for the third consecutive year, the UK failed to reach 95 per cent coverage for under-fives, missing out on the global target set by the World Health Organisation.