Originally published 2022-06-23.
In June 2022 the UK Health Security Agency (UKHSA) announced that poliovirus had been found in sewage samples from the Beckton Sewage Treatment Works in east London, and that the pattern suggested the virus was spreading between people in parts of north and east London. Health officials treated it as a national incident. It was not, however, a problem with London’s drinking water, and no one in the UK was diagnosed with polio as a result.
The virus was a vaccine-derived type 2 strain, not wild polio. The overall risk to the public was described as extremely low, because most people in Britain were vaccinated as children. The practical message was simple: make sure children’s polio vaccines are up to date. By August, every child aged 1 to 9 in London was being offered an extra dose.
What was actually found
The UK monitors sewage routinely for poliovirus. It is common for one or two vaccine-like viruses to turn up each year, usually shed for a short time by someone recently vaccinated abroad with the oral vaccine. Those detections normally appear once and disappear.
This time was different. Samples taken between February and May 2022 contained several closely related viruses, and they kept appearing. Genetic links between them meant the virus was being passed from person to person, most likely among closely connected people in a few neighbourhoods, and was evolving as it went. By then it had changed enough to be classed as vaccine-derived poliovirus type 2, known as VDPV2.
Context matters here. The last case of polio caught from wild virus inside the UK was confirmed in 1984, and the country was declared polio-free in 2003. Finding the virus in wastewater was a warning signal, not a count of sick people.
What “vaccine-derived” means
There are two main kinds of polio vaccine. The injected inactivated vaccine (IPV), first developed by Jonas Salk’s team in the early 1950s, uses killed virus and cannot cause infection. The oral vaccine (OPV), developed by Albert Sabin and in use from 1961, contains a weakened live virus. It is cheap, easy to give with drops in the mouth and excellent at stopping spread, which is why it is still used in global eradication campaigns.
The weakened virus in the oral vaccine is shed in stools for a few weeks. Where many people are unvaccinated, it can pass between them for months and slowly mutate back towards a form that can paralyse. That does not harm the child who was vaccinated; the danger is to unprotected people further down the chain. The UK uses only the injected vaccine, so the virus in London must have been brought in by someone vaccinated with the oral vaccine elsewhere and then spread locally.
Fully vaccinated people are protected against paralytic disease from vaccine-derived strains just as they are against wild ones.
What health officials said
Dr Vanessa Saliba, a consultant epidemiologist at UKHSA, stressed that this kind of virus was uncommon and that the general public was at very low risk. She told Sky News: “Vaccine-derived poliovirus is rare and the risk to the public overall is extremely low.” Her concern was for communities where fewer children were vaccinated, and she urged parents who were unsure to check their child’s red book, the personal health record given to UK families, or ask their GP.
Professor Adam Finn of the Joint Committee on Vaccination and Immunisation explained why experts suspected local spread. One-off detections from a recently vaccinated traveller tend to vanish quickly, he said, but finding genetically related viruses again and again over several months strongly pointed to transmission inside London.
The NHS was asked to report any suspected case quickly, and the NHS in London began contacting parents of children under five who had missed doses.
What happened next
Wider testing confirmed the picture. By 10 August 2022, UKHSA had identified 116 type 2 poliovirus isolates in 19 sewage samples collected between 8 February and 5 July. Most were still vaccine-like, and a smaller number had drifted far enough to count as vaccine-derived. Positive samples came from parts of eight boroughs: Barnet, Brent, Camden, Enfield, Hackney, Haringey, Islington and Waltham Forest, areas with some of the lowest vaccination rates in the country.
On the advice of the JCVI, every child aged 1 to 9 in London was offered a booster of the inactivated vaccine, starting in the affected boroughs, even if they were already up to date. Sewage sampling was expanded to more sites in London and around the country. UKHSA also worked with health agencies in New York and Israel and with the World Health Organization, after related polio detections were reported there.
No paralytic case was confirmed in the UK, and the episode is often cited as an example of wastewater surveillance working as an early warning.
Polio in brief
Polio is caused by a virus that spreads mainly through contact with infected faeces, for example via unwashed hands, and less often through droplets. Most infections cause no symptoms or a mild illness. In a small minority the virus attacks nerves in the spinal cord, causing paralysis that can be permanent or, if breathing muscles are affected, fatal.
There is no cure, only prevention. Global vaccination since 1988 has cut cases by more than 99 percent. Wild type 2 virus has been declared eradicated, and wild polio now circulates only in Pakistan and Afghanistan. The newer oral vaccine nOPV2, which is far less likely to revert, has been rolled out since 2021 to help close the vaccine-derived gap.
Common myths
“Polio was in London’s tap water.” No. The samples were raw sewage at a treatment works, collected for monitoring. Treated drinking water is a separate system.
“The vaccine gives you polio.” The injected vaccine used in the UK cannot. The rare problem comes from the live oral vaccine spreading in poorly vaccinated communities, which is why high coverage matters.
“Adults vaccinated as children need a booster.” UK advice in 2022 was that most adults were already protected. Anyone unsure about their own or a child’s vaccination history, or planning travel to an affected country, should talk to a doctor or practice nurse.
Sources: UKHSA, 22 June 2022: Poliovirus detected in sewage from north and east London; UKHSA, 10 August 2022: All children aged 1 to 9 in London to be offered a dose of polio vaccine; Sky News, June 2022; Wikipedia: Polio vaccine.
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