Public health officials have detected poliovirus—the virus that can cause a disabling, life-threatening disease in humans—in sewage samples in London. Though the overall risk to the public is low and no cases of polio have been reported so far, top doctors in the United Kingdom are urging parents to ensure their children are vaccinated against the virus.
U.K. health officials regularly analyze wastewater as part of a routine surveillance program, according to a statement from the U.K. Health Security Agency (UKHSA). They usually have one to three poliovirus detections in the country’s sewage every year, but these are usually one-off findings that quickly dissipate and are likely shed by someone who was recently vaccinated overseas with the live oral polio vaccine, which produces a weakened polio virus that only causes disease in exceptionally rare cases.
But this spring, health officials have been tracking “several closely related viruses” repeatedly detected in sewage samples from February through May, according to the statement. These repeat findings suggest that the virus is continuing to evolve and may be spreading between “closely linked individuals” in north and east London, according to the UKHSA. Epidemiologists are classifying the virus as a “vaccine-derived poliovirus type 2” (VDPV2), which is rare but can cause serious symptoms in unvaccinated individuals. No actual cases of paralysis or illness have been reported, according to the World Health Organization (WHO).
The virus likely came from an individual who arrived in the country in early January, reports the New York Times’ Apoorva Mandavilli and Euan Ward. Four of the latest samples likely evolved from that initial virus in unvaccinated children.
"It sounds like the outbreak is very small," Angela Rasmussen, a virologist at the University of Saskatchewan, tells NPR’s Michaeleen Doucleff. "The outbreak could be within an extended family. Transmission would require a concentration of people who had not yet been vaccinated."
Health officials are now expanding sewage surveillance to understand the virus’ transmission and localization. The wastewater treatment facility where the samples were collected serves more than half of the city’s population—about 4 million people—so identifying the virus’ origins could prove challenging.
In the U.K. and the United States, individuals are vaccinated using an inactivated form of poliovirus, which does not present in their feces. But other parts of the world use a live, weakened version of the virus. People inoculated with this type of vaccine can briefly shed traces of the virus in their feces.
The virus collected from London’s sewage probably came from a person who was recently in Afghanistan, Pakistan or some countries in the Middle East and Africa, where the live vaccine is common, per the New York Times. Vaccine-derived polio, like the virus found in London, can cause small outbreaks, hence the announcement from the U.K. government.
“Vaccine-derived poliovirus has the potential to spread, particularly in communities where vaccine uptake is lower,” says Vanessa Saliba, an epidemiologist with the UKHSA, in the statement. “On rare occasions, it can cause paralysis in people who are not fully vaccinated … Most of the U.K. population will be protected from vaccination in childhood, but in some communities with low vaccine coverage, individuals may remain at risk.”
Health officials in the U.K. are now reaching out to parents of kids under age 5 in London who need to get their vaccines updated. Though the WHO considers the U.K. to be polio-free, with a low risk of transmission because of high vaccination rates, childhood vaccination coverage has been decreasing in recent years, according to the statement. Per the WHO, an estimated 86.6 percent of people in London are vaccinated against polio.
The last case of wild polio in the U.K. was reported in 1984. Before the polio vaccine, outbreaks were common in the country, with some 8,000 instances of paralysis caused by the virus reported annually during the 1950s. In the U.S., the disease disabled more than 35,000 individuals each year in the late 1940s, according to the U.S. Centers for Disease Control and Prevention (CDC). After a widespread vaccination campaign, the number of polio cases worldwide numbers usually in the teens in the 21st century.
The virus spreads via person-to-person contact, either from contact with the feces of an infected person or from droplets expelled from a sneeze or cough. About 25 percent of people who come down with the virus will experience flu-like symptoms such as fever, sore throat and nausea, which usually go away on their own within two to five days.
A much smaller share of infected individuals–about one to five in 1,000—will experience more serious symptoms, including paralysis, meningitis (an infection of the spinal cord and brain) and paresthesia (the feeling of pins and needles in their legs).
In the U.S., children are typically vaccinated against polio as babies, and then again around school age.