LONDON (AP) — For years, health officials worldwide have used billions of drops of an oral vaccine in a remarkably effective campaign aimed at eradicating polio in the last remaining strongholds — typically poor, 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 uncovered evidence that polio is spreading there.
The original source of the virus? The oral vaccine itself.
Scientists have known about this extremely rare phenomenon for a long time. That’s why some countries have switched to other polio vaccines. But these occasional infections of the oral formula are becoming more and more noticeable as the world moves closer to eradication of the disease and the number of polio cases caused by the wild or naturally circulating virus falls.
According to figures from the World Health Organization and its partners, there have been 396 cases of polio caused by the wild virus since 2017, compared to more than 2,600 related to the oral vaccine.
“We’re basically replacing the wild virus with the virus in the vaccine, which is now leading to new outbreaks,” said Scott Barrett, a professor at Columbia University who has studied polio eradication. “I assume that countries like the UK and the US can stop the transfer pretty quickly, but we thought the same about monkey pox.”
The latest incidents represent the first time in years that the vaccine-bound polio virus has emerged in wealthy countries.
Earlier this year, officials in Israel discovered polio in an unvaccinated 3-year-old, who was paralyzed. Several other children, almost all unvaccinated, were found to have the virus but no symptoms.
In June, British authorities said they had found evidence in the sewer that the virus was spreading, although no human infections were identified. Last week, the government said all children between the ages of 1 and 9 in London would receive a booster shot.
In the US, an unvaccinated young adult suffered paralysis in his legs after being infected with polio, New York officials revealed last month. The virus has also surfaced in New York’s sewers, suggesting it’s spreading. But officials said they are not planning a booster campaign because they believe the state’s high vaccination rate should provide adequate 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 per 2 million doses. (It takes four doses to be fully immunized.) In extremely rare cases, the attenuated 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 have been vaccinated pass live virus from the vaccine in their stool. From there, the virus can spread within the community and, over time, turn into a form that can paralyze people and trigger new epidemics.
Many countries that have eradicated polio switched decades ago to injectable vaccines containing a killed virus to avoid such risks; the Scandinavian countries and the Netherlands have never used the oral vaccine. The ultimate goal is to get the whole world moving once wild polio is eradicated, but some scientists argue the switch should happen sooner.
“Without the (oral polio vaccine) we probably could never have conquered polio in the developing world, but this is the price we are paying now,” said Dr. Paul Offit, director of the Children’s Center Vaccine Education Center. Philadelphia Hospital. “The only way we can eradicate polio is by eliminating the use of the oral vaccine.”
Aidan O’Leary, director of the WHO’s polio division, described the discovery of the spread of polio 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 been killed for immunizing children and where conflict has blocked access to some areas.
Still, O’Leary said he is confident Israel, Britain and the US will soon stop their newly identified outbreaks.
The oral vaccine is credited with dramatically reducing the number of children paralyzed by polio. When global eradication efforts began in 1988, there were approximately 350,000 cases of wild polio each year. There have been 19 cases of wild polio so far this year, all in Pakistan, Afghanistan and Mozambique.
In 2020, vaccine-related polio cases peaked at over 1,100 across dozens of countries. Since then, that has dropped to about 200 this year so far.
Last year, WHO and partners also began using a newer oral polio vaccine, which contains a live but attenuated virus that scientists believe is less likely to mutate into a dangerous form. But supplies are limited.
More vaccination is needed to stop polio in Britain, the US and Israel, experts say. That’s something Columbia University’s Barrett concerns could be challenging in the COVID-19 era.
“What’s different now is reduced trust in authorities and political polarization in countries like the US and the UK,” Barrett said. “The assumption that we can quickly increase vaccination rates may now be more challenging.”
Oyewale Tomori, a virologist who helped direct Nigerian efforts to eradicate polio, said he and his colleagues have been hesitant to describe outbreaks as “vaccine derived” because they feared people would become 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 retrospect, it might have been better not to use this vaccine, but at the time no one knew it was going to end like this.”
The Associated Press Health and Science Department is supported by the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.