A polio outbreak in a war-torn country could lead to the re-introduction of the virus to Europe, experts say.

Once on the brink of global eradication, the polio virus has re-emerged in Syria, prompting a massive regional vaccination effort.

The vaccination campaign, announced Friday by UNICEF and the World Health Organization (WHO), aims to inoculate 20 million Middle Eastern children against the disease. Also Friday, the European Center for Disease Prevention and Control released a statement reiterating the steps it is taking to halt the spread of the virus into that region.

Both developments emerged the day after a German professor, Martin Eichner of the University of Tübingen, and Stefan Brockmann, of Germany’s Reutlingen Regional Public Health Office published a letter in The Lancet saying more needs to be done to keep the virus from spreading.

“Vaccinating only Syrian refugees—as has been recommended by the European Center for Disease Prevention and Control—must be judged as insufficient; more comprehensive measures should be taken into consideration,” they wrote.

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So far, 10 cases of polio paralysis have been confirmed among children in Syria, where vaccination levels have fallen to 68 percent. The U.S. has been without indigenous polio cases since 1979, according to the U.S. Centers for Disease Control and Prevention.

Until recently, polio only remained endemic in Pakistan, Nigeria, and Afghanistan. “It is disturbing that polio still has some hiding places,” Eichner told Healthline. “The global eradication campaign has done a great job in fighting it back, and we have been on the verge of getting rid of it for years now. Unfortunately, some countries are lacking enough vaccination. For some, this may be because they assume that they are safe from polio; for others, like Syria, it is simply because their public health structure has suffered from turmoil.”

The European disease control agency has called for increased environmental monitoring throughout the continent and has stepped up vaccination efforts in countries where the overall vaccination rate is below 90 percent, especially in Eastern Europe.

Although only 10 cases of polio paralysis have been confirmed in Syria, the actual number of children carrying the virus could be much higher.

“The vast majority of polio virus infections do not cause any symptoms at all,” Eichner said. “Only about one in 200 infections of previously unvaccinated individuals causes paralysis. This, of course, is good for the individual carrying the virus, but on the other hand, it is a public health nightmare to have people spread a potentially dangerous infection without being recognized.”

In the Lancet piece, Eicnher wrote that it could take one year of “silent transmission” before a single case of polio paralysis is detected in Europe. By that time, hundreds of people would be carrying the infection, he explained.

“A lot of people and officials have been worried about this happening,” Eichner said. “There are many countries where the vaccination coverage is down to such a low degree that it must simply be regarded as a lot of luck that they have escaped introduction of the polio virus.”

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The oral polio vaccine is more effective at preventing transmission of the virus, but is not used in most developed countries. Most Western nations, including the U.S., prefer the inactive vaccine. The active, oral vaccine comes with a risk of paralysis in one out of every 2.4 million vaccinated patients; the inactive version does not.

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Eichner said the inactive vaccine is generally appropriate. But in countries that are seeing an influx of Syrian refugees or have poor hygiene and crowding problems, the oral vaccine is necessary.

In a joint statement, UNICEF and the World Health Organization said that UNICEF has procured 1.35 billion doses of oral polio vaccine so far, and that by the end of the year they will have procured up to 1.7 billion doses to meet increased demand.

“Global supply of [oral vaccine] was already under constraint, with vaccine manufacturers producing at full capacity,” the statement reads. “The new outbreak in Syria is adding further pressure to the supply, but WHO, UNICEF, and manufacturers are working to secure sufficient quantities to reach all children.”

In a statement to Healthline, the U.S. Centers for Disease Control said the agency continues to work with international partners to eradicate polio at its source. “The potential risk of transmission to [Europe] and elsewhere documents the need for strong ongoing global efforts to eradicate this disease. Polio anywhere should be considered polio everywhere.”

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