The United Kingdom has detected a potential outbreak of polio for the first time in almost 40 years—and while the risk to the general public remains low, health authorities are encouraging those who are unvaccinated, particularly young children, to get their shots as soon as possible.

Poliovirus detected in London sewage

According to the New York Times, the last known case of polio in Britain was in 1984, and the country has been polio-free since 2003. However, the U.K. Health Security Agency (UKHSA) recently discovered poliovirus in several sewage samples from North and East London in February and April, suggesting that there may have been transmission between individuals.

The virus detected in the samples has been classified as a "vaccine-derived" poliovirus type 2. In this case, the original virus likely came from someone who was vaccinated with live oral polio vaccine, which can then "shed" traces of the virus in feces. Currently, only Afghanistan, Pakistan, and some Middle Eastern and African countries continue to use this type of polio vaccine, which can sometimes lead to small outbreaks.

Based on a genetic analysis, the samples share a common origin, most likely from someone who traveled to Britain around the beginning of the year, and the virus has continued to evolve since it first began spreading. Community transmission is likely occurring among unvaccinated young children, but it is also possible that an immunocompromised individual has been shedding traces of the virus for months, the Times reports.

"It sounds like the outbreak is very small," said Angela Rasmussen, a virologist at the University of Saskatchewan. "The outbreak could be within an extended family. Transmission would require a concentration of people who had not yet been vaccinated."

Currently, U.K. health authorities have declared the discovery of poliovirus a "national incident" and are working to expand surveillance of sewage samples to pinpoint the origin of the outbreak and determine whether it is still ongoing.

"We are urgently investigating to better understand the extent of this transmission," said Vanessa Saliba, an epidemiologist who consults for UKHSA.

Health officials encourage vaccination

So far, no cases of polio have been reported, and UKHSA said the risk to the general public is extremely low, largely due to high vaccination rates. According to the Global Polio Eradication Initiative, recent data shows that polio vaccination rates in London are around 86.6%.

However, some communities with low polio vaccination rates may be at risk of infection, and UKHSA recommends that anyone who is not vaccinated do so soon. In particular, Jane Clegg, chief nurse for the National Health Service, said the agency plans to contact parents of children under five who are living in London and not yet vaccinated. Although most cases of polio are asymptomatic, it can cause permanent paralysis in up to 1% of unvaccinated patients.

According to Rasmussen, disrupted childhood vaccination programs during the pandemic likely contributed to poliovirus's reemergence and could lead to more outbreaks. Between 2019 and 2020, the number of polio cases worldwide nearly tripled to more than a thousand cases across almost 30 countries, and last year, there were more than 600 cases of polio, NPR reports.

"My biggest concern is the fact that there is a larger population now that hasn't been vaccinated on schedule," Rasmussen said. "I think that applies everywhere. And that's because of the SARS-CoV-2 pandemic."

In the United States, polio vaccination is part of routine childhood immunizations, and CDC recommends children receive four doses of the inactivated polio vaccine at two months, four months, six months, and four to six years of age.

"If you're not up to date on your polio vaccine, now's the time to go out and get up to date," Rasmussen said. (Mandavilli/Ward, New York Times, 6/22; Scribner, Axios, 6/22; Gans, The Hill, 6/22; Doucleff, "Goats and Soda," NPR, 6/22; Alund, USA Today, 6/22)