For years, global health officials have used billions of drops of an oral vaccine in a remarkably effective campaign aimed at eradicating polio in its last remaining strongholds, usually poor and politically unstable corners of the world.
Now, in a surprising twist in the decades-long effort to eradicate the virus, authorities in Jerusalem, New York and London have discovered evidence that polio is spreading there.
The original source of the virus? The oral vaccine itself.
Scientists have long known about this extremely rare phenomenon. This is why some countries have switched to other polio vaccines. But these incidental infections from oral formula are becoming more and more glaring as the world moves closer to eradication of the disease and the number of polio cases caused by the wild or naturally circulating virus plummets.
Since 2017, there have been 396 cases of polio caused by the wild virus, compared to more than 2,600 linked to the oral vaccine, according to figures from the World Health Organization and its partners.
“We’re basically replacing the wild virus with the vaccine virus, which is now causing new outbreaks,” said Scott Barrett, a Columbia University professor who has studied polio eradication. “I would assume that countries like the UK and the US will be able to stop transmission quite quickly, but we also thought about monkeypox.”
The latest incidents represent the first time in several years that the vaccine-related polio virus has appeared in rich countries.
Earlier this year, officials in Israel detected polio in an unvaccinated 3-year-old boy, who suffered paralysis. Several other children, almost all unvaccinated, were found to have the virus but no symptoms.
In June, British authorities reported finding evidence in sewage that the virus was spreading, although no human infections were identified. Last week, the government said all children in London between the ages of 1 and 9 will be offered a booster shot.
In the United States, an unvaccinated young adult suffered paralysis in his legs after being infected with polio, New York officials revealed last month. The virus has also appeared in the sewers of New York, suggesting it is spreading. But officials said they don’t plan a booster campaign because they believe the state’s high vaccination rate should provide enough protection.
Genetic analyzes showed that the viruses in the three countries were all “vaccine-derived,” meaning they were mutated versions of a virus that originated in the oral vaccine.
The oral vaccine in question has been used since 1988 because it’s cheap, easy to administer—two drops are put directly into children’s mouths—and better at protecting entire populations where polio is spreading. It contains a weakened form of the live virus.
But it can also cause polio in about two to four children for every 2 million doses. (Four doses are required to be fully immunized.) In extremely rare cases, the weakened virus can also sometimes mutate into a more dangerous form and cause outbreaks, especially in places with poor sanitation and low vaccination levels.
These outbreaks usually start when people who are vaccinated shed the live virus from the vaccine in their stool. From there, the virus can spread within the community and eventually become a form that can paralyze people and start new epidemics.
Many countries that eliminated polio switched to injectable vaccines containing a virus killed decades ago to avoid these risks; the Nordic countries and the Netherlands never used the oral vaccine. The ultimate goal is to move the entire world to the shots once wild polio is eradicated, but some scientists argue that the change should happen sooner.
“We probably never would have been able to overcome polio in the developing world without the (oral polio vaccine), but that’s the price we’re paying now,” said Dr. Paul Offit, director of the Center for Education for Children’s vaccines. Philadelphia Hospital. “The only way to eliminate polio is to eliminate the use of the oral vaccine.”
Aidan O’Leary, director of WHO’s polio department, described the discovery of polio spreading in London and New York as “a big surprise”, saying officials have focused on eradicating the disease in Afghanistan and Pakistan, where health workers have died. to immunize children and where the conflict has made access to some areas impossible.
Still, O’Leary said he’s confident Israel, Britain and the US will quickly shut down their newly identified outbreaks.
The oral vaccine is credited with dramatically reducing the number of children paralyzed by polio. When the global eradication effort began in 1988, there were about 350,000 cases of wild polio per year. So far this year, there have been 19 cases of wild polio, all in Pakistan, Afghanistan and Mozambique.
In 2020, the number of vaccine-related polio cases peaked at more than 1,100 spread across dozens of countries. It has since dropped to about 200 this year so far.
Last year, the WHO and its partners also began using a new oral polio vaccine, which contains a live but weakened virus that scientists believe is less likely to mutate into a dangerous form. But supplies are limited.
To stop polio in Britain, the United States and Israel, what is needed is more vaccination, experts say. That’s something Columbia University’s Barrett worries could be a challenge in the age of COVID-19.
“What’s different now is a reduction in trust in the authorities and political polarization in countries like the US and the UK,” Barrett said. “The presumption that we can rapidly increase vaccination numbers may be more difficult now.”
Oyewale Tomori, a virologist who helped lead Nigeria’s effort to eliminate polio, said that in the past he and his colleagues were reluctant to describe outbreaks as “vaccine-driven,” distrusting that people were afraid of the vaccine.
“All we can do is explain how the vaccine works and hope people understand that immunization is the best protection, but it’s complicated,” Tomori said. “In hindsight, it might have been better not to use this vaccine, but at the time, no one knew it would turn out like this.”