The United Kingdom is on high alert after polio virus was found in its sewage system. Public health officials in the country are now urging anyone who is not fully immunized to get vaccinated immediately.

The World Health Organization said it confirmed that “vaccine-derived poliovirus type 2” was found in environmental samples in London. As The Wall Street Journal explained, that’s “a mutated form of the weakened live virus found in the oral polio vaccine still used in some countries. People who receive the oral vaccine shed small amounts of virus in their feces. In places that lack adequate sanitation, it sometimes mutates to resemble the naturally occurring virus and can infect people who aren’t fully immunized.”

U.K. health officials emphasize they haven’t identified any polio cases, though the sewage samples indicate local spread of the virus.

Wednesday, the U.K. Health Security Agency issued a press release announcing the discovery and launch of an investigation. In it, Dr. Vanessa Saliba, consulting epidemiologist for the agency, said that vaccine-derived poliovirus is rare and the risk to the public overall is extremely low. But she noted the virus has the potential to spread, especially in areas where vaccine rates are lower. Those who are vaccinated don’t need to worry, but others should get the vaccine, she said.

The Wall Street Journal reported that “the detection of polio in the West is very rare and represents the latest setback for the global push to eradicate the disease, which mostly affects children and in rare, severe cases, can cause paralysis. Global health organizations say that the eradication of polio is within reach, but that task has been frustrated by reappearance of the virus in countries where authorities thought it had been beaten back.”

The last case found in Britain was in 1984. The World Health Organization declared the U.K. polio-free in 2003. The United States got that designation after its last case in 1979.

While largely eradicated worldwide, The Washington Post noted that polio remains endemic in Afghanistan and Pakistan.

About polio

Polio — its full name is poliomyelitis — is a very contagious virus that is spread through contact with feces and, more rarely, respiratory secretions of an infected person. It lives in the digestive system and throat and attacks the central nervous system. Those most at risk are children under 5.

The Centers for Disease Control and Prevention says 72% of those infected won’t have any symptoms, while 1 in 4 will will have flu-like symptoms including sore throat, fever, tiredness, nausea, headache and stomach pain. Typically, symptoms resolve quickly on their own.

Between 1 and 5 of 1,000 of those infected develop more serious symptoms that impact the brain and spinal cord. Those symptoms can include:

  • Parasthesia, which is a sense of pins and needles in the legs.
  • Meningitis, an infection of the covering of the spinal cord and/or the brain.
  • Paralysis, which can lead to permanent disability. If the paralysis impacts muscles used to breathe, 2%-10% of people who develop paralysis die.

Children who appear to recover fully sometimes experience new muscle pain, weakness or paralysis 15 to 40 years later. That’s called post-polio syndrome.

Infected individuals can spread poliovirus immediately before and up to two weeks after they notice symptoms. Those without symptoms can spread it, too. The virus can live in an infected person’s feces for many weeks and, without diligent hygiene, the infection can spread to unvaccinated people, according to health experts.

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The vaccines

In the early 1950s, polio outbreaks caused more than 15,000 cases of paralysis a year, according to the CDC.

The first polio vaccine was developed in 1956. Today, the polio vaccine is deemed 99%-100% effective against the virus.

The most common form of polio is called wild polio, but the BBC explains that there’s a rarer type linked to the oral form of the vaccine, which is the suspect in the London sewage discovery.

BBC says the oral vaccine provides excellent protection against wild polio, is easy to use and has been deployed by many countries around the world, keeping millions safe. “However, it contains a weakened, live form of the virus which can replicate harmlessly in the gut. But that means some is then excreted in poo. In rare cases, this weakened form can spread to unvaccinated people. Over a long period the vaccine-derived virus might change to become more like wild polio.”

That’s one reason why many industrialized countries now use the newer injectable vaccine with killed virus. Both are safe and effective, the article said.

A few countries, including Afghanistan, Pakistan and Nigeria, continue to use the oral vaccine. And the risk is small.

The BBC reports that in the last decade, with more than 10 billion doses of oral polio vaccine administered, vaccine-derived-polio virus outbreaks resulted in fewer than 800 cases.

“In the same period, in the absence of vaccination with the oral polio vaccine, more than 6.5 million children would have been paralyzed by wild poliovirus,” the article said.

How polio was detected in sewage

U.K. officials said routine surveillance of sewage there finds the virus a couple of times a year, but this spring officials found the virus in several samples collected in London.

“Genetic analysis suggests that the samples have a common origin, most likely an individual who traveled to the country around the new year,” The New York Times reported, citing Dr. Shahin Huseynov, a technical health officer for the World Health Organization. “The last four samples collected appear to have evolved from this initial introduction, likely in unvaccinated children.”

While officials are trying to identify the source of the virus, it’s not going to be easy: The area that funnels waste into the plant includes 4 million people.

Stopping the spread

The Global Polio Laboratory Network said it’s important that all countries, but especially those with a lot of travel and contact with polio-infected countries, “strengthen surveillance in order to rapidly detect any new virus importation and to facilitate a rapid response.”

The U.K. said it plans to push parents of young children to get vaccinated if they are not already. The World Health Organization says the polio vaccination rate in London is nearly 87%.

Such a small outbreak could be within an extended family, virologist Angela Rasmussen, of the University of Saskatchewan in Canada, told NPR.

She said the COVID-19 pandemic increased the number of people who are not vaccinated. In 2016, there were a few dozen polio cases across four countries, but in 2019-2020, cases nearly tripled. That resulted in more than a dozen known cases spread across 30 countries, NPR reported.