Spread of polio shows rare risk of oral vaccine


For years, global health officials have used billions of drops of the oral vaccine in a remarkably effective campaign to eradicate polio in its last remaining strongholds – typically poor, politically unstable corners of the world.

Now, in a surprising twist in a decades-long effort to eradicate the virus, authorities in Jerusalem, New York and London have discovered evidence that polio is widespread there.

Origin of the virus? Self-medication.

Scientists have known about this extremely rare phenomenon for a long time. That’s why some countries have turned to other polio vaccines. But these accidental infections from oral formula are becoming more apparent as the world moves closer to eradicating the disease and the number of spontaneous, or naturally occurring, viral polio cases decreases. strong.

Since 2017, there have been 396 cases of polio caused by the rabies virus, compared with more than 2,600 cases linked to the oral vaccine, according to figures from the World Health Organization and its partners. this position.

“We’re essentially replacing the wild virus with the vaccine virus, which is currently leading to outbreaks,” said Scott Barrett, a Columbia University professor who has worked on eradicating polio. new. “I think countries like the UK and the US will be able to end transmission pretty quickly, but we’ve also been thinking about monkeypox.”

The latest incidents show that for the first time in several years the vaccine-associated polio virus has emerged in rich countries.

Earlier this year, officials in Israel discovered polio in an unvaccinated, paralyzed three-year-old child. Several other children, nearly all of whom were unvaccinated, were found to have the virus but had no symptoms.

In June, British authorities said they had found evidence in sewage that the virus was spreading, although no cases of human infection had been identified. Last week, the government said all children in London between the ages of one and nine would be given a booster shot.

In the US, an unvaccinated young man became paralyzed in his legs after being infected with polio, New York officials revealed last month. The virus has also appeared in New York sewers, suggesting it is spreading. But officials said they are not planning an increased campaign because they believe the state’s high vaccination rates will provide enough protection.

Genetic analyzes show that the viruses in the three countries are all “vaccine-derived”, meaning they are mutated versions of a virus derived from an oral vaccine.

The oral vaccine in question has been in use since 1988 because it is cheap, easy to use – two drops are put directly into children’s mouths – and better protects entire populations where polio is spreading. It contains a live attenuated form of the virus.

But it can also cause paralysis in about two to four children per two million doses. (Four doses are required to be fully immunized.) In extremely rare cases, a weakened virus can sometimes mutate into a more dangerous form and cause disease outbreaks, especially in places with poor sanitation and low vaccination levels.

These outbreaks usually begin when vaccinated people shed live virus from the vaccine in their feces. From there, the virus can spread in the community and over time will turn into a form that can paralyze people and start new diseases.

Many countries that eliminated polio have switched to injectable vaccines containing the virus that was destroyed decades ago to avoid such risks; Nordic countries and the Netherlands never used oral vaccines. The ultimate goal is to switch the entire world to vaccinations once wild polio is eradicated, but some scientists think the switch should happen sooner.

Dr Paul Offit, director of the Children’s Vaccine Education Center, said: “We may never get over polio in the developing world without (oral polio vaccine). ), but this is the price we are paying. Philadelphia Hospital. “The only way we’re going to eliminate polio is by eliminating the use of oral vaccines.”

Aidan O’Leary, director of the WHO’s polio division, described the discovery of widespread polio in London and New York as “a big surprise”, saying officials were focused on eradicating the disease in Afghanistan and Pakistan, where medical workers have been killed. to vaccinate children and when conflict has made access to some areas impossible.

However, O’Leary said he was confident Israel, Britain and the US would quickly quell their newly identified outbreaks.

Oral vaccines are thought to have significantly reduced the number of children who are paralyzed by polio. When the global eradication effort began in 1988, there were about 350,000 cases of wild polio each year. To date, 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 cases spread across dozens of countries. Since then, it has dropped to about 200 this year so far.

Last year, WHO and partners also started using a newer oral polio vaccine, containing a live but attenuated virus that scientists believe is less likely to mutate into a form of polio. hazardous. But supplies are limited.

To prevent polio in the UK, US and Israel, more vaccinations are needed, experts say. That’s what Barrett of Columbia University worries could be challenging in the COVID-19 era.

“What’s different now is a decline in the trust of the authorities and political polarization in countries like the US and UK,” Barrett said. “The assumption that we can rapidly increase the number of vaccines quickly may be more difficult than it is now.”

Oyewale Tomori, a virologist who helped direct Nigeria’s polio eradication effort, said he and colleagues had previously tried to describe outbreaks as “vaccine-derived.” “, wary that it will scare people about vaccines.

“All we can do is explain how vaccines work and hope that people understand that vaccination is the best protection, but it’s complicated,” Tomori said. “In hindsight, it was probably better not to use this vaccine, but at the time, nobody knew it would turn out like this.”

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