“Confounding,” “very unorthodox,” and “very concerning.” That’s how Shahin Huseynov, polio lead for the World Health Organization’s (WHO) European region, describes the slew of detections over the past 3 months of poliovirus in wastewater in Spain, Poland, and Germany. On 10 December, the U.K. government reported more positive wastewater samples, as did Finland. So far, no paralytic cases have been found, and the risk of an outbreak in these countries with high vaccination rates is deemed fairly low.
Huseynov is one of dozens of researchers in Europe and the United States trying to figure out where the poliovirus came from and how it is popping up so widely in such a short time. “Genetically, the picture is very, very unusual. It is very fascinating,” Huseynov says. The isolates are descended from a strain circulating in Africa, he says, but the extent of their sequence variation from that strain—and from each other—make it hard to reconstruct where the virus first arrived in Europe, and whether it was imported multiple times or was transmitted locally after a single introduction.
All the newly spotted isolates are vaccine-derived strains, known as circulating vaccine-derived poliovirus type 2, which are spawned by the live, attenuated virus used in the oral polio vaccine (OPV). They are descended from the Zamfara strain, which emerged in Nigeria several years ago and has spread widely across Africa. The closest match is to the strain now circulating in Algeria, Guinea, and Mali, says Robb Butler, director of communicable diseases for the WHO European region.
Usually, by analyzing sequence variation among virus isolates, genetic epidemiologists can piece together where a poliovirus originated and the route of its spread. But in this case, “the genetic data does not provide a clear picture,” Huseynov says. Genetic analysis shows the virus has been circulating undetected for about 1 year—somewhere. He thinks Africa is most likely, because high-quality surveillance in Europe would have detected such extended and wide circulation. “But we can’t completely rule out that it happened in Europe. Anything is possible,” he says.
What’s crystal clear, however, is it is spreading fast.
The mystery began in Barcelona, Spain, in mid-September, when routine wastewater sampling turned up a single isolate of a circulating vaccine-derived poliovirus type 2. These emerge when the virus used in OPV circulates for extended times in areas of low immunization and regains its ability to spread and to cause paralysis. Sporadic detections of these vaccine-derived viruses are not uncommon in polio-free countries, brought in by people traveling from, or who have recently visited, an OPV-using country. OPV is still used in Africa to battle outbreaks and in Afghanistan and Pakistan, where the wild virus remains endemic. But Europe, the U.S., and much of the world stopped using OPV decades ago and replaced it with the inactivated polio vaccine (IPV), a killed vaccine that cannot revert to its virulent state. These outbreaks typically die out, as occurred in the U.S., Israel, and the United Kingdom in 2022.
A vaccine-derived poliovirus type 2 was then detected in a sewage sample collected on 22 October in Warsaw, Poland. Next it showed up in Germany in samples collected over a 2-week span in Munich, Cologne, Bonn, and Hamburg. Last week, wastewater samples tested positive in Dresden, Dusseldorf, and Mainz. This week, wastewater samples in England’s Leeds, London, and West Sussex collected in November tested positive. Yesterday, Finland reported it had isolated a vaccine-derived poliovirus type 2 in wastewater in Tampere.
Although the virus was first detected in Spain, “it’s not possible … to assess the relative time of introduction in each of the countries,” says Sabrina Bacci, head of vaccine-preventable diseases and immunization at the European Centre for Disease Prevention and Control (ECDC).
Overall, polio vaccination rates are high in the European countries that have detected the isolates. But pockets of under- or unvaccinated children exist in every country, so the risk of paralytic cases remains, Bacci says. Spread to Ukraine, Bosnia, and Herzegovina is a huge worry, because low vaccination rates there could fuel a sustained outbreak and, potentially, paralytic cases.
The detections in seven cities in Germany would seem to suggest the virus is now being transmitted locally there. (IPV protects against paralysis, but when infected, vaccinated people can still transmit the virus, which is shed in their feces.) In an unusual twist, however, the isolates from the seven German cities are more closely related to those from Spain and Poland than they are to each other. “That’s why we can’t confirm local transmission in Germany, and we can’t deny it,” Huseynov says.
His “working hypothesis” is that multiple, concurrent importations of the virus into Europe is the most plausible explanation. Ondrej Mach, who heads polio research and product development at the Global Polio Eradication Initiative (GPEI) headquartered at WHO, thinks there might have been a couple of importations from Africa, but since then, the virus has likely established local transmission.
Spain, Poland, and Germany have all stepped up their wastewater surveillance. The three countries are also reviewing vaccination records to find any immunity gaps, and they are on the lookout for paralytic cases that may have been missed. The Public Health Agency of Catalonia has reported that no poliovirus has been found in wastewater since the initial September detection. “The countries are doing the right things,” says Oliver Rosenbauer, a spokesperson for GPEI. “It’s great they detected a public health risk before it has had the chance to cause any actual disease.”
ECDC has urged other European countries to intensify surveillance. But just 26 of the 53 countries in the region now routinely sample wastewater, Butler says, and that means the poliovirus could be anywhere.
Just like in 2022, Mach says, the most likely scenario is that the virus will die out in Europe. But if poliovirus circulation continues, some experts say European countries might need to consider adding a novel form of OPV, known as nOPV2, along with IPV, because of its proven ability to halt transmission. That would be a controversial move, considered a step backward, for a region in which most countries stopped using the oral vaccine almost 20 years ago after it was declared polio-free.
Rosenbauer, like others, emphasizes that reinfections with wild or vaccine-derived poliovirus will continue “because this highly infectious disease spreads so easily and silently across wide geographic areas,” he says. “What this really speaks to is the need to eradicate polio everywhere.”
