Yesterday, declaring a “national incident” after poliovirus was detected in London sewage, public health officials in the United Kingdom asked physicians to be on alert for polio cases and urged residents to check whether they are up to date with their vaccinations for the now- rare disease. The source of the virus is still a mystery, but it was likely someone from outside the United Kingdom who had recently received the oral polio vaccine (OPV), which uses a live, but weakened, virus and is no longer given in the United Kingdom .
Most polio infections are asymptomatic, and no cases have been found in the United Kingdom so far. But with vaccination rates below 90% in some London communities, there is concern that won’t last. The appearance of poliovirus in London is a reminder “that no country is immune from being reinfected by polio or it re-emerging, until the disease has been eradicated everywhere,” says Oliver Rosenbauer, a spokesperson for the World Health Organization’s Global Polio Eradication Initiative .
Polio has been eliminated in much of the world, but the virus is still endemic in Afghanistan and Pakistan, and 30 more countries in Africa, Europe, and the Middle East are now designated “outbreak countries,” where the virus has circulated recently. The spread is seeded either by the wild virus that persists in Afghanistan or Pakistan or by virus from OPV that has regained an ability to cause disease in unvaccinated people.
In the United Kingdom, the virus was detected in samples taken between February and June from the Beckton Sewage Treatment Works, which processes wastewater from 4 million people in northern and eastern London. It did not initially raise alarm; sewage-monitoring programs in the UK usually detect a few such samples each year, shed by recent arrivals who have received OPV.
Such sewage traces usually disappear again fairly quickly. This time, the virus continued to appear in London over several months, and several closely related versions of the virus turned up in recent samples. The genetic changes suggest the virus is continuing to evolve, a sign that it may be spreading in a small number of people.
Joseph Swan, a polio communications officer at WHO, says broader spread is unlikely, given the generally high vaccine coverage in the UK, which like most wealthy countries uses multiple doses of a killed-virus vaccine. And he emphasizes there is not yet direct evidence that the virus is spreading from person to person. But officials are encouraging people to check that they and their children have received all recommended vaccine doses.
The London polio detection is the second this year in a place that doesn’t usually have the virus. On 7 March, an unvaccinated 3-year-old in Israel who had developed paralysis was diagnosed with polio, and the country has detected poliovirus in 25 sewage samples so far this year, mostly in or near Jerusalem. The virus there is also a vaccine-derived strain, but it is the type 3 strain, unrelated to the type 2 virus detected in London. The rapid detection in both Israel and the United Kingdom demonstrates that polio surveillance systems there are working, Rosenbauer says.
Israel and the Palestinian National Authority have ramped up vaccination efforts in response, and the virus hasn’t been detected in sewage there since March. But COVID-19 has made the job harder, says Itamar Grotto, an infectious disease epidemiologist at Ben-Gurion University of the Negev. Both the public and public health staff are worn down after the pandemic and multiple COVID-19 vaccination campaigns, he says, and in some groups vaccine hesitancy has increased.
Officials in London are now taking samples in sewer systems upstream from the Beckton treatment plant, to see whether they can narrow down the source of the virus and perhaps target vaccination campaigns more precisely.
The OPV that brought the virus to both the United Kingdom and Israel has advantages in regions where poliovirus is spreading. Not only is it cheap and easy to administer, but also the live virus in the vaccine can still replicate in a recipient, triggering strong immune protection in the gut that can stop further transmission of the virus, helping boost community defenses against future outbreaks. And although it occasionally evolves to regain its virulence, health officials are starting to roll out a more stable form of OPV that they hope will reduce the risk of these reversions.
In the meantime, Rosenbauer says, “The main thing countries can do… is to ensure strong disease surveillance and high vaccination coverage, to minimize the risk and consequences of re-emergence.”