An 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.

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.

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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.