It has been nearly 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 largely been eliminated. Afghanistan and Pakistan are the only two countries left in the world where polio is still classified as endemic.
But occasional outbreaks remain common – and now the UK has become the latest country to report what appears to be a recurrence of the deadly disease.
Here’s what we know:
How many cases were detected?
At this stage it is unclear. In an announcement on Wednesday, the UK Health Safety Agency (UKHSA) said the virus was identified in sewage samples collected from London between February and May 2022.
The agency said it was “likely” that the virus was spreading between “closely linked” individuals in northeast London, and an investigation had been launched to establish whether any community transmission was taking place.
So far, no suspected cases have been reported or confirmed. Neither were any associated cases of paralytic poliomyelitis detected.
The samples were collected at the London Beckton Sewage Works, which covers a population of approximately four million people.
What do we know about viral samples?
Analysis shows that the virus picked up by UKHSA is not ‘wild type’ polio. Instead, it is a version of the virus that derives from the live oral polio vaccine (OPV).
Live OPV is used in poorer parts of the world to respond to polio outbreaks. This vaccine generates intestinal immunity and for several weeks after vaccination, people can shed the vaccine virus in their stool.
These vaccine viruses can spread in undervaccinated communities from person to person through poor hand hygiene and contamination of water and food. Coughing and sneezing, although less common, is another route of transmission.
As it spreads, as appears to have been the case in Britain over the course of several months, the virus may mutate into a vaccine-derived version of the poliovirus, which behaves more like the naturally occurring ‘wild’ type.
These strains of the virus are known as vaccine-derived poliovirus type 2 (VDPV2), which is what the UKHSA detected in London sewage samples.
How does polio spread?
The virus spreads in poorly vaccinated communities from person to person through poor hand hygiene and contamination of water and food. Coughing and sneezing, although less common, is another route of transmission.
An estimated 95 to 99 percent of people who contract poliovirus are asymptomatic. Even without symptoms, people infected with poliovirus can still spread the virus and cause infection in others.
In the case of the UK, it appears that the vaccine-derived virus was introduced earlier this year and started to spread between February and May.
It is likely that the virus was introduced to the UK by a person vaccinated with OPV in a foreign country. From there, it may have spread within a single family.
“These findings suggest that there may be localized spread of poliovirus, likely in individuals who are not current on polio immunizations,” said Dr. Kathleen O’Reilly, associate professor of statistics for infectious diseases.
How dangerous is polio?
Most people infected with the virus (about 72 out of 100, according to the US Centers for Disease Control and Prevention) will not develop any visible symptoms, meaning the infection comes and goes without any harm.
About 1 in four will have flu-like symptoms such as a sore throat, fever, tiredness, nausea, headache and stomach pain. These usually persist for 2 to 5 days.
A smaller proportion of people (1-5 in 1,000) with a poliovirus infection will develop other, more serious symptoms that affect the brain and spinal cord, leading to paralysis in some cases.
Paralysis is the most serious symptom associated with polio, as it can lead to permanent disability and death. Between 2 and 10 out of every 100 people who suffer from paralysis from poliovirus infection die, because the virus affects the muscles that help them breathe.
Even children who seem to make a full recovery can develop new muscle pain, weakness, or paralysis as adults, 15 to 40 years later. This is called post-polio syndrome.
How is polio stopped?
Vaccination is the most effective way to reduce transmission in a community and protect people.
The UK switched from using OPV to an inactivated polio vaccine (IPV), given by injection, in 2004. Injections are given in routine NHS childhood vaccines at 8, 12 and 16 weeks as part of vaccine 6 in 1. Boosters are offered at age three and age 14.
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. Both nations rely on OPV for their immunization programs.