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n outbreak of vaccine-related polio cases has been reported in war-torn Syria, the World Health Organization announced Thursday — another hurdle for a nearly 30-year eradication effort that has seen more than its share of setbacks.

Two children in the Deir-Ez-Zor governorate in eastern Syria have been paralyzed by the vaccine viruses. The virus has been recovered from a third child who was not paralyzed. All signs point to vaccine viruses that are circulating in the area.

Detailed planning is already underway for emergency vaccination programs to reach about 200,000 children in the region, Michel Zaffran, head of the WHO’s polio program, told STAT. The government of Syria has made a formal request for the emergency vaccination effort, he said.

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Given the six-year-old civil war in that country, the endeavor will be logistically challenging. But Syria experienced a polio outbreak in 2013 and 2014, and it was extinguished.

In that case, the outbreak was caused by so-called wild polioviruses. The new cases are vaccine-derived polioviruses — viruses from the oral polio vaccine used in some developing countries — and not an indication that the viruses are spreading as they are in Afghanistan and Pakistan, believed to be the only two countries in the world where wild polioviruses still circulate.

“It’s still complex,” Zaffran admitted.

The Global Polio Eradication Initiative has a long history of operating in places of political instability and has been successful in the past in negotiating so-called “days of tranquility” in which ceasefires allow vaccination teams to reach otherwise hard-to-reach children.

In mid-May, an anonymous contributor reported to the online disease surveillance system ProMED that there was a cluster of children who had suffered what’s called acute flaccid paralysis in Syria. Sudden onset of paralysis can be caused by a number of things, but in parts of the world where polio is still a threat, it must be ruled out when such cases arise.

The polio program has received reports of 58 children in the region who have recently experienced acute flaccid paralysis. Testing showed the vaccine viruses were not responsible in 11 of these cases, but further testing remains to be done on samples from the others.

Getting samples from Deir-Ez-Zor to laboratories in Damascus and Ankara that can do the testing isn’t easy or quick.

“This is still a work in progress,” Zaffran said. “We have samples that have reached the lab that have tested negative. We have samples that have reached the lab and have not yet been completely tested. And we have samples that have been collected that are still stuck in Syria and are not yet transported.”

Oral polio vaccine is made using live but weakened viruses; the original formulation contained components to protect against all three polioviruses, types 1 through 3. Type 2 polioviruses stopped circulating nearly 20 years ago.

While highly effective, the oral vaccine has some rare but serious side effects. The weakened viruses can spread from a vaccinated child to other children with whom he or she is in contact, also passively vaccinating them. But as the viruses spread from child to child they can mutate to become virulent again, regaining the power to cripple.

That’s what has happened in Syria. And there may be more cases, Zaffran acknowledged.

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Genetic analysis shows the viruses from the three children are related and have been circulating for a period of about two years, Zaffran said. The fact that vaccine viruses have been circulating for that long suggests a lot of children in the region have not been successfully immunized against polio.

“That can only happen in populations that have low immunity,” he said.

The type 2 component of the oral polio vaccine is the most likely of the three to regain virulence. As a result, last year in a globally coordinated maneuver called “the switch” all countries that use oral vaccine had to stop administering the three-component type and adopt a version that only protects against type 1 and type 3 polioviruses.

But supplies of a special monovalent or single-strain vaccine that protects against type 2 polio have been stockpiled in case outbreaks like the type in Syria crop up. This is the vaccine that will be used in this emergency response.

Zaffran said response planners are hoping to move quickly but must ensure that the effort is effective at reaching as many children as possible.

That’s because they know putting more type 2 vaccine viruses into an environment where there are unimmunized children risks igniting future outbreaks like this one.

“Speed is relatively important but quality is going to be even more important,” Zaffran said. “We want to really interrupt the transmission, protect the population and ensure that we don’t seed more of these … [outbreaks]. So it’s going to be important to do it very properly.”

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  • Is it vaccines what the children in Syria really need?? A reasonable person would say (in addition to cessation of war of course) it’s first and foremost proper nourishment and shelter. Provide them with food and roof first.

  • This is sad news but we are hopeful of winning war against Polio. This should not deter the course of Polio Erandication endgame strategy

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