It has been almost 40 years since the UK detected its last outbreak of poliovirus, an infection feared for its ability to leave patients paralyzed and crippled.
Thanks to extensive vaccination efforts around the world, the disease has been largely eradicated. Afghanistan and Pakistan are the only two remaining countries in the world where polio is still classified as endemic.
However, occasional outbreaks are still common – and now the UK is the latest country to report what appears to be a resurgence of the deadly disease.
Here’s what we know:
At this stage it is not clear. In an announcement published on Wednesday, the UK Health Security Agency (UKHSA) said the virus had been identified in sewage samples collected in London between February and May 2022.
The agency said it was “likely” that the virus had spread between “closely linked” people in north-east London and an inquiry had been launched to see if community transmission was taking place.
So far, no suspected cases have been reported or confirmed. No associated cases of paralytic polio have been identified either.
Samples were collected from London Beckton Sewage Works, which has a population of approximately four million people.
Analysis shows the virus discovered by the UKHSA is not ‘wild-type’ polio. Instead, it is a version of the virus derived from the live oral polio vaccine (OPV).
Live OPV is being used in poorer parts of the world to respond to polio outbreaks. This vaccine creates gut immunity, and for several weeks after vaccination, people can shed the vaccine virus in their stool.
These vaccine viruses can then spread from person to person in undervaccinated communities through poor hand hygiene and contamination of water and food. Coughs and sneezes, although less common, are another route of transmission.
As it spreads, as it appears to have done in the UK over several months, the virus can mutate into a vaccine-derived version of the poliovirus that behaves more like the naturally occurring ‘wild-type’.
These virus strains are known as vaccine-derived poliovirus type 2 (VDPV2), which the UKHSA has identified in London sewage samples.
The virus spreads from person to person in poorly vaccinated communities through poor hand hygiene and water and food contamination. Coughs and sneezes, although less common, are another route of transmission.
It is estimated that 95 to 99 percent of people who contract the poliovirus are asymptomatic. Even without symptoms, people infected with the poliovirus can spread the virus and cause infection in others.
In the case of the UK, the vaccine-derived virus appears to have been introduced earlier this year and spread between February and May.
It is likely that the virus was introduced into the UK by a person who was vaccinated with OPV in an overseas country. From there it may have spread within a single family.
“These results suggest that poliovirus can spread locally, most likely in individuals who are not up to date on polio vaccination,” said Dr. Kathleen O’Reilly, Associate Professor of Infectious Disease Statistics.
Most people infected with the virus (about 72 in 100, according to the US Centers for Disease Control and Prevention) develop no visible symptoms, meaning the infection comes and goes without harm.
About 1 in 4 have flu-like symptoms such as a sore throat, fever, tiredness, nausea, headache and abdominal pain. These usually last 2 to 5 days.
A smaller proportion of people (1-5 in 1,000) with poliovirus infection develop other, more serious symptoms that affect the brain and spinal cord, and in some cases lead to paralysis.
Paralysis is the most serious symptom associated with polio, as it can lead to permanent disability and death. Between 2 and 10 out of 100 people who become paralyzed from poliovirus infection die because the virus attacks the muscles that help them breathe.
Even children who appear to make full recoveries can develop new muscle pain, weakness, or paralysis 15 to 40 years later than adults. This is called post-polio syndrome.
Vaccination is the most effective way to contain transmission in a community and protect people.
The UK switched from using the OPV to an inactivated polio vaccine (IPV) given by injection in 2004. Vaccinations are given as part of routine NHS childhood vaccinations at 8, 12 and 16 weeks as part of the 6-in-1 vaccine. Boosters are offered at ages three and 14.
The last case of wild polio in the UK was confirmed in 1984 and Britain was declared virus-free in 2003. Afghanistan and Pakistan are the only two countries in the world where the infection is still classified as endemic. Both nations rely on the OPV for their vaccination programs.