A microscopic view of poliovirus Stocktrek Images, Inc./Alamy
Poliovirus has been detected in a number of London sewage samples, prompting the UK Health Security Agency (UKHSA) to urge anyone who isn’t up to date with their polio vaccines to get immunised.
The samples, detected at Beckton sewage treatment works between February and May, have so far been limited to wastewater, with no reports of human cases.
Nevertheless, some degree of local transmission is expected to have taken place, with the virus having the potential to spread and cause severe disease, particularly among the unvaccinated.
What is polio and how does it spread?
Polio is a viral disease that mostly affects children under 5. It usually spreads by people not washing their hands properly after using the toilet and then contaminating food or drink that is consumed by someone else. In rare cases, it can spread by coughing and sneezing.
What are the symptoms?
Most people infected with polio have no symptoms, but some develop flu-like symptoms such as a high temperature and vomiting within a few weeks of being infected.
In every 1 in 100 to 1 in 1000 cases, the virus can attack spinal nerves, causing paralysis. Polio can also be deadly if it affects the nerves that control our breathing muscles.
When was the UK’s last polio outbreak?
No cases of polio have been reported in the UK since 1984. The World Health Organization subsequently declared the UK polio-free in 2003.
Is it unusual to detect polio in wastewater?
Sewage samples are regularly tested in the UK for poliovirus and other pathogens, such as norovirus and hepatitis A and E.
One to three “vaccine-like” polioviruses are detected each year. These were previously one-off findings that occurred after a person received a live oral polio vaccine and shed the virus in their faeces. These oral vaccines aren’t administered in the UK.
In the Beckton samples, several closely related polioviruses have been identified.
“It is likely there has been some spread between closely-linked individuals in north and east London and that they are now shedding the type 2 poliovirus strain in their faeces,” according to the UKHSA.
This strain could cause infection, but probably not severe disease, in people who have received the injected polio vaccine. Used in the UK, this contains a “killed” version of the virus.
How can people shed poliovirus in their faeces?
Live oral polio vaccines haven’t been used in the UK since 2004, but are given in parts of the world that are actively fighting the infection, such as Afghanistan and Pakistan. Unlike vaccines that are administered via injection, oral vaccines are relatively cheap and easy to administer.
The major disadvantage of the oral vaccine is it can lead to vaccine-derived poliovirus in poorly immunised populations. This vaccine works by administering a weakened version of the live poliovirus. This brings about an infection, but has been modified so as not to cause severe disease, except in rare cases. The virus then replicates in the intestine, which can cause it to be excreted via faeces.
In regards to the Beckton sewage samples, the strain found was probably introduced by someone who travelled to the UK after recently receiving the live oral vaccine.
What makes this different to previous polio detections in sewage?
Previous detections of poliovirus in sewage samples occurred independently as separate cases.
In the Beckton samples, the viruses were identified across a four-month period. They are also closely related to each other and contain mutations that suggest they are evolving.
According to David Salisbury at the World Health Organization’s Global Commission for Certification of Polio Eradication, these mutations “imply that it has circulated amongst individuals, including possibly those who have been vaccinated with inactivated polio vaccine”.
This circulation may have occurred in the country where the person received the live oral vaccine or possibly in the UK, but without causing symptoms.
While no human cases have been reported so far in the UK, Kathleen O’Reilly at the London School of Hygiene & Tropical Medicine expects some local transmission has taken place undetected.
“With the sewage samples, the genetic analysis indicates multiple chains of transmission, which suggests some people are spreading it among themselves,” she says. “People who have never been vaccinated have a much higher chance of acquiring the infection and shedding it for a long period of time.”
Are only unvaccinated people at risk?
The risk of serious disease from polio is low in the UK, according to the UKHSA. “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,” Vanessa Saliba at UKHSA said in a statement.
In the UK, an injected vaccine is given to babies three times before they turn 1. Two more booster jabs are then administered before they turn 15.
More than 92 per cent of the UK population has received at least three polio vaccine doses, but take-up is lower in London at about 86 per cent.
“In populations with low vaccine uptake it is possible that the live polio vaccine can spread from one person to another,” said Paul Hunter at the University of East Anglia, UK, in a statement to the Science Media Centre.
Overall, how concerned should we be?
“There have not been any paralytic cases so far,” said Hunter. “So at the moment there is unlikely to be any immediate risk to public health, but if such transmission continues then the risk is that the virus will eventually evolve into one that does cause paralysis.
“If that does happen, then this could pose a serious risk to people who have not been vaccinated. Such vaccine-derived transmission events are well described and most ultimately fizzle out without causing any harm, but that depends on vaccination coverage being improved.”
Jonathan Ball at the University of Nottingham in the UK said the disease threat in the country is “low” due to our relatively widespread use of the injected vaccine, “but we might see some continued spread of the vaccine strain as killed vaccine doesn’t always protect from infection”.
“Ultimately, though, this virus should disappear because of the high levels of vaccination here,” he told the Science Media Centre.
The NHS will contact the parents of children under 5 years in London who aren’t up to date with their polio vaccines, urging them to get immunised.
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