The discovery of poliovirus in New York state, London and Jerusalem this year 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,” said Zulfikar Bhuta, a global health researcher at the Aga Khan University in Karachi, Pakistan. “Everything is interconnected.”

The virus found in these regions originated from an oral polio vaccine used in some countries. So far, only two cases of polio-related paralysis have been reported, in Jerusalem in February and in New York in June; the New York infection was the first such case in the US in nearly a decade. But sewage samples in all three areas suggest the virus is circulating more widely.

Polio causes irreversible paralysis in fewer than one in 200 of the susceptible people it infects, so the cases of paralysis suggest that many other people out there have been infected, says Walter Orenstein, who studies infectious diseases at Emory University in Atlanta, Georgia. . “Cases like this are just the tip of the iceberg,” he says. “It’s very worrying.”

Nature talks to researchers about the scale of the epidemic and what can be done to stop it.

Why do these outbreaks happen?

Wild poliovirus circulates in only two countries — Afghanistan and Pakistan — where nine cases were reported this year through June.

But vaccine-derived poliovirus appears periodically elsewhere, particularly in Africa and Asia. These cases come from a widely used oral vaccine that contains a live, weakened virus that sometimes mutates to a dangerous form capable of infecting the nervous system.

Neither the United States nor the United Kingdom use this vaccine, opting instead for an injectable vaccine containing inactivated virus. This vaccine can prevent the virus from infecting the nervous system, but it is not as effective as the oral virus at reducing virus shedding and stopping transmission, says Raoul Andino-Pavlovsky, a microbiologist at 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 spared the worst effects of the virus (about 94 percent of 5- and 6-year-olds in the U.S. are vaccinated). But unvaccinated people are vulnerable to the disease.

“This virus is very, very good at finding unvaccinated individuals,” Orenstein says. In the 1990s, a poliovirus outbreak in the Netherlands established a foothold in a community with a relatively low vaccination rate, despite the country’s overall vaccination coverage of more than 90%, says Oliver Rosenbauer, spokesman for the Global Polio Eradication Initiative Health Organization (WHO) in Geneva, Switzerland. The outbreak resulted in 2 deaths and 59 cases of paralysis and occurred 14 years after the country’s last endemic polio case.

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

Sewage surveillance for poliomyelitis is rare in rich countries; The UK only routinely monitors sewage for the disease in London and Glasgow. And New York City began testing wastewater in July after officials learned of a case of polio-related paralysis.

So it’s unclear how far the virus may have spread. But there’s reason to hope it hasn’t gone far: in London, which since February found poliovirus in the sewage, the virus appears to have remained concentrated in the north and east of the city, and there have been no reports of polio-related paralysis. “It seems to be quite localized, even within London,” says infectious disease epidemiologist Nicholas Grassley of Imperial College London.

In New York, the virus has been found in two counties as well as in New York City, a geographic spread that is “very concerning,” Orenstein said. “That suggests we’ve had significant transmission.”

The US Centers for Disease Control and Prevention (CDC) will prioritize the monitoring of poliomyelitis wastewater in undervaccinated communities associated with the paralysis case. “Testing for poliovirus from sewage samples is very labor intensive and resource intensive,” said a CDC spokesperson Nature. But surveillance systems set up to monitor the SARS-CoV-2 coronavirus are working to provide support, she added.

Can outbreaks be contained?

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

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

If, after about six months, testing of the wastewater shows that the poliovirus has continued to spread, other options may need to be considered, he says. For example, in 2020 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 an error-prone enzyme involved in genome replication can generate genetic changes — to create a set of mutations that prevent the virus from regaining its ability to infect the nervous system. “It’s like putting the virus in an evolutionary cell,” says Andino-Pavlovsky, who helped design the vaccine.

This vaccine has not yet undergone large-scale human testing and has not been approved by UK or US regulatory authorities. But more than 100 million people have gotten it, Andino-Pavlowski says, without any signs of vaccine-derived polio.

What is the status of polio outbreaks elsewhere?

Outbreaks in rich countries get a lot of attention, Rosenbauer says. “But in the last 20 years, we’ve had very, very serious outbreaks around the world in developing countries.”

Rosenbauer says there are encouraging signs that the battle to eradicate polio is progressing, despite the disruption caused by COVID-19. He says outbreaks in Yemen and several countries in Africa are still active but shrinking in scope.

In places where there is armed conflict, such as areas of Afghanistan and Yemen, vaccination is difficult. “We need political will to implement a plan to reach all children in difficult areas,” he says. Otherwise, “the disease will return globally.”

This article is reproduced with permission and was first published on August 22, 2022

https://www.scientificamerican.com/article/new-polio-outbreaks-worldwide-put-scientists-on-alert/

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