Scientists are on alert as polio outbreaks continue to rise around the world


The virus in the live oral vaccine widely used for polio can on rare occasions mutate into a dangerous form capable of infecting the nervous system.Credit: Ezra Akayan/Getty

The discovery of poliovirus in New York state, London and Jerusalem this year has surprised many — but public-health researchers fighting to eradicate the disease say it was only a matter of time.

“No country in the world is immune to the effects of polio,” says Zulfiqar Bhutta, a global-health researcher at the Aga Khan University in Karachi, Pakistan. “It’s all interconnected.”

The virus found in these areas originated from the oral polio vaccine used in some countries. Only two cases of polio-related paralysis have been reported so far, in Jerusalem in February and New York in June.1; The New York infection was the first such US case in nearly a decade. But wastewater samples from all three areas show the virus is spreading more widely.

Walter Orenstein, who studies infectious diseases at Emory University in Atlanta, Georgia, says that polio causes irreversible paralysis in about one in 200 people who are likely to be infected, so cases of paralysis indicate that many more people are infected. “Such cases are just the tip of the iceberg,” he says. “It’s very worrying.”

Nature Talked to researchers about the extent of the outbreak, and what can be done to prevent it.

Why are these disasters happening?

Wild poliovirus has spread to only two countries – Afghanistan and Pakistan – where nine cases were reported in June this year.

But vaccine-derived polioviruses occasionally appear elsewhere, particularly in Africa and Asia. These cases come from a widely used oral vaccine that contains a live, weakened virus that sometimes mutates into a dangerous form that can infect the nervous system.

Neither the United States nor the United Kingdom use that vaccine, opting instead for an injectable vaccine containing an inactivated virus. The vaccine may prevent the virus from infecting the nervous system, but it is not as effective as the oral virus at reducing viral shedding and preventing transmission, says microbiologist Raul Andino-Pavlovsky of the University of California, San Francisco.

High rates of polio vaccination in the United Kingdom, Israel, and the United States mean that most children will be protected from the worst effects of the virus (about 94% of US 5- and 6-year-olds are vaccinated). But unvaccinated people are at risk of the disease.

“This virus is very good at finding people who are unvaccinated,” says Orenstein. A poliovirus outbreak in the Netherlands in the 1990s established a community with relatively low vaccination rates despite the country’s overall vaccination coverage exceeding 90%, says Oliver Rosenbauer, spokesman for the Global Polio Eradication Initiative. Health Organization (WHO) in Geneva, Switzerland. The outbreak caused 2 deaths and 59 cases of paralysis, and was the last endemic case of polio in the country for 14 years.

Has poliovirus spread beyond the area where it was first identified?

Wastewater surveillance for polio is rare in rich countries; The United Kingdom routinely monitors sewage for disease only in London and Glasgow. And New York began testing wastewater in July, after officials learned of polio-related paralysis.

So it is unclear how far the virus may have spread. But there’s reason to hope it hasn’t gone too far: In London, where poliovirus has been detected in sewage since February, the virus appears to be concentrated in the north and east of the city, and there have been no reports of polio. associated paralysis. “Even within London it seems to be fairly localized,” says Nicholas Grassley, an infectious-disease epidemiologist at Imperial College London.

In New York, the virus has been detected in two counties as well as New York City, a geographic spread that is “very alarming,” Orenstein says. “That suggests we’ve had enough transmission.”

The US Centers for Disease Control and Prevention (CDC) will prioritize wastewater surveillance for polio in communities with low vaccination rates linked to polio cases. “Poliovirus testing from wastewater samples is very labor and resource-intensive,” said a CDC spokesperson. Nature. But surveillance systems set up to monitor the coronavirus SARS-CoV-2 are working to provide support, he added.

Can the outbreak be controlled?

The United States, Israel and the United Kingdom are all boosting vaccination efforts, which should fill the gaps created during the COVID-19 pandemic. This includes an ambitious effort to vaccinate all children aged one to nine in London.

This strategy, says Grassley, should nip the outbreak in the bud. But the campaigns in London and New York will use injectable vaccines, so they won’t stop the transmission of the virus.

If, in six months or so, tests of the wastewater show that poliovirus continues to spread, other options may need to be looked at, he says. For example, in 2020, the WHO listed a new oral polio vaccine for emergency use.

This vaccine contains a weakened poliovirus. But the researchers used knowledge of the virus’s genome — including how the error-prone enzymes involved in replicating the genome can cause genetic changes — to create a suite of mutations that prevent the virus from regaining the ability to infect the nervous system. “It’s like putting a virus in an evolutionary cage,” says Andino-Pavlovsky, who helped design the vaccine.

That vaccine has not yet undergone large-scale human trials, and has not been approved by UK or US regulators. But more than 100 million people who have received it, says Andino-Pavlovsky, have shown no symptoms of vaccine-derived poliovirus.

What is the status of polio outbreaks elsewhere?

Disasters get more attention in rich countries, says Rosenbauer. “But in the last 20 years, we’ve had many, very serious outbreaks around the world in developing countries.”

Rosenbauer says there are encouraging signs that the fight to eradicate polio is making progress, despite the setbacks caused by COVID-19. Outbreaks are still active in Yemen and some countries in Africa, he says, but their scope is shrinking.

Vaccination is difficult in places of armed conflict, such as Afghanistan and Yemen. “Political will is needed to implement plans to reach all children in challenging areas,” he says. Otherwise, “the disease will return globally”.

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