Dr. Martin Eichner of the University of Tubinge in Germany and Dr. Stefan Brockmann of the Reutlingen Regional Public Health Office wrote Thursday in a letter to the journal The Lancet hundreds of thousands of people are fleeing Syria and seeking refuge in neighboring countries and Europe.
Polio can spread silently from person-to-person -- only 1-in-200 unvaccinated individuals infected with wild-type poliovirus 1 will develop acute flaccid paralysis -- but infected individuals can spread the virus unrecognized. In about 1 percent of cases, the virus enters the central nervous system, infecting and destroying motor neurons, leading to muscle weakness and paralysis.
Polio is primarily caused by a virus that spreads among people, but it can also survive in water and sewage.
It could take a year of silent transmission before one acute case of paralysis case is confirmed and an outbreak detected, although hundreds could carry the infection, the researchers wrote.
The inactivated polio vaccine, which is used throughout Europe, only partly prevents vaccinees from infection, but it reduces transmission and is highly effective in prevention of paralysis.
In regions with low vaccination coverage, e.g., Bosnia and Herzegovina at 87 percent, Ukraine at 74 percent or Austria at 83 percent not enough of people are inoculated so herd immunity -- enough people are vaccinated or immune so those at risk have low risk of being infected -- might be insufficient to prevent sustained transmission, the researchers said.
"Vaccinating only Syrian refugees -- as has been recommended by the European Centre for Disease Prevention and Control -- must be judged as insufficient; more comprehensive measures should be taken into consideration," the researchers said in their letter. "Oral polio vaccination provides high protection against acquisition and spreading of the infection, but this vaccine was discontinued in Europe because of rare cases of vaccination-related acute flaccid paralysis. Only some of the European Union member states still allow its use and none has a stockpile of oral polio vaccines."
In addition, routine screening of sewage for poliovirus has not been done in most European countries, but this intensified surveillance measure should be considered for settlements with large numbers of Syrian refugees, the researchers said.
Wild poliovirus had not been detected in Syria since 1999 and prior to the conflict, immunization coverage in Syria was about 95 percent, UNICEF officials said.
About 500,000 children in Syria have not been vaccinated against polio in the past two years due the fighting, health officials said.
In October, health authorities in Syria, the World Health Organization, UNICEF and neighboring countries had begun planning and implementing a comprehensive immunization program to vaccinate children against polio, measles, mumps and rubella, in both government-controlled and contested areas. About 2.4 million children d younger age 5 were scheduled to be vaccinated.
However, in an environment of widespread displacement and continual population movements, even a single polio case poses a serious threat across the region, UNICEF said.