Poliovirus has been found in several London sewage samples, prompting the UK Health Security Agency (UKHSA) to urge anyone who has not received their polio vaccines to do so.
The samples, discovered between February and May at the Beckton sewage treatment works, have so far been limited to wastewater, with no reports of human cases.
Despite this, some local transmission is expected, with the virus having the potential to spread and cause severe disease, particularly among the unvaccinated.
What is polio and how does it spread?
What are the symptoms?
When was the UK’s last polio outbreak?
Is it unusual to detect polio in wastewater?
How can people shed poliovirus in their feces?
What makes this different from previous polio detected in sewage?
This spread could have occurred in the country where the person received the live oral vaccine, or it could have occurred in the UK, but without causing symptoms.
While no human cases have been reported in the UK so far, Kathleen O'Reilly of the London School of Hygiene and Tropical Medicine believes that some local transmission has occurred undetected.
"The genetic analysis of the sewage samples shows multiple chains of transmission, indicating that some people are spreading it among themselves," she says. "People who have never been immunized have a much higher risk of contracting the infection and shedding it for an extended period of time."
Are only unvaccinated people at risk?
According to the UKHSA, the risk of serious disease from polio is low in the UK. "The majority of the UK population will be protected from childhood vaccination, but individuals may remain at risk in some communities with low vaccine coverage," Venessa Saliba of UKHSA said in a statement.
In the United Kingdom, babies are given an injected vaccine three times before the age of one. Before they turn 15, they will receive two more booster shots.
More than 92% of the UK population has received at least three doses of the polio vaccine, but take-up is lower in London, at around 86%.
"It is possible that the live polio vaccine can spread from one person to another in populations with low vaccine uptake," said Paul Hunter of the University of East Anglia in the UK in a statement to the Science Media Centre.
How concerned should we be?
"There have been no paralytic cases so far," Hunter said. "At the moment, there is unlikely to be any immediate threat to public health, but if such transmission continues, the virus may eventually evolve into one that causes paralysis."
"If that occurs, it could pose a serious risk to people who have not been immunized." Such vaccine-derived transmission events are well described and will most likely fade away without causing any harm, but this is dependent on increased vaccination coverage."
According to Jonathan Ball of the University of Nottingham in the United Kingdom, the disease threat in the country is "low" due to our relatively widespread use of the injected vaccine, but "we may see some continued spread of the vaccine strain as the killed vaccine does not always protect from infection."
"However, because of the high levels of vaccination here, this virus should eventually disappear," he told the Science Media Centre.
The NHS will contact parents of children under the age of five in London who have not received their polio vaccines, urging them to do so.