Experts have raised the prospect of polio returning to Europe as a result of the Syrian crisis.

The continent was declared polio-free by the World Health Organisation (WHO) in 2002, but is now threatened by a new
outbreak of the disease reported in Syria, it is claimed.

But academics have warned that refugees fleeing the war-torn country could bring the disease back.

Two German infection experts writing in The Lancet medical journal highlight the fact that the polio vaccine currently used throughout Europe is not 100% effective.

In parts of Europe where vaccine coverage is low, including Bosnia and Herzegovina, Ukraine, and Austria,
immunisation levels may not be high enough to prevent sustained transmission of the polio virus, they stress.

The WHO has confirmed an outbreak of at least 10 cases of polio in Syria, where vaccination coverage has dramatically
decreased because of the civil war.

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With large numbers of people now fleeing Syria and seeking refuge in neighbouring countries and Europe, polio could reappear in areas that have been free of the disease for decades.

"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," said Professor Martin Eichner, from the University of Tubingen, and Dr Stefan Brockmann, from the Department of Infection Control in Reutlingen.

The polio vaccine used in most European countries today is the inactivated polio vaccine (IPC), which is injected. It usually forms part of a combined diphtheria, tetanus, whooping cough and polio jab.

While good at preventing paralysis, IPC provides only partial protection from infection, Prof Eichner and Dr Brockmann point out.

Once children were routinely vaccinated with an oral polio vaccine (OPV), taken in the form of mouth drops, which uses a live virus and is more effective. But this was discontinued in the UK and many other countries because in rare cases it can trigger paralysis.

Since only one in 200 polio infections cause symptoms, the virus could be circulating for nearly a year before a single case of paralysis occurs, according to the experts. By this time, hundreds of individuals may be carrying the virus.

Prof Eichner and Dr Brockmann wrote: "Hundreds of thousands of people are fleeing Syria and seek refuge in neighbouring countries and Europe. Because only one in 200 unvaccinated individuals infection with WPV1 (wild-type polio virus 1) will develop acute flaccid paralysis, infected individuals can spread the virus unrecognised.

"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. Routine screening of sewage for polio virus has not been done in most European countries, but this intensified s
urveillance measure should be considered for settlements with large numbers of Syrian refugees."

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