UK health officials said Wednesday that "several" polioviruses were found in sewage samples between February and May.
The find is raising concern that polio may have spread "between closely-linked individuals" in North and East London.
No cases have been confirmed, and health officials say the risk to the public "is extremely low."
The UK Health Security Agency says it's possible that there may be "some spread" of polio happening in small clusters in London.
Wastewater samples taken from North and East London between February and May suggest that the crippling disease, which used to kill hundreds of people in the UK every year, could be circulating "between closely-linked individuals."
Experts are still cautious about this sewage finding, though, stressing that no polio cases have been confirmed in humans in the UK yet, so the extent of any polio outbreak — and whether one even exists — is unclear.
However, the presence of "several" different sewage samples that look quite similar to one another, virologically speaking, is giving investigators pause. (When people have polio, they can shed traces of it in their poop, so the contaminated sewage is a sign that some spread between people may already be happening in London.)
Viral shedding explains why polio might be in the sewers
While wild polio has been near-eradicated around the world through widespread vaccination, people who've been recently immunized against the disease in countries where live polio vaccines are still used can shed the virus for a little while, as a result of their successful vaccination.
That viral shedding (through stool, or respiratory secretions like sneezes) can, in turn, infect unprotected individuals who haven't been vaccinated yet.
"On rare occasions it can cause paralysis in people who are not fully vaccinated, so if you or your child are not up to date with your polio vaccinations it's important you contact your GP," Dr. Vanessa Saliba with the UKHSA said Wednesday in a statement.
It's important to note that the UK does not use live poliovirus vaccines, so the spread of polio from vaccinated people is not normally a concern there. But, on occasion, people who've traveled to the UK from overseas and who were recently vaccinated against polio with a live virus vaccine have shed polio into UK sewage before. The UKHSA says in the past, those were always "one-off findings that were not detected again," making the current cluster of positive samples more concerning.
"We are urgently investigating to better understand the extent of this transmission, and the NHS has been asked to swiftly report any suspected cases to the UKHSA," Saliba said.
Polio can infect a person's spinal cord, making it crippling, or deadly, but many people show no outward symptoms of the virus, and it's possible to recover from some flu-like symptoms of an infection within a few days.
However, even people who make a full recovery from polio can go on to develop paralysis decades later in life, with post-polio syndrome.
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