Vaccinating only Syrian refugees against polio may not be enough to prevent the crippling viral disease from re-infecting Europe where it has not been seen for decades, German scientists warned on Friday.

Writing in The Lancet medical journal, they said the risk to Europe from a re-emergence of polio in Syria was partly due to the type of vaccine generally used in regions that have not had the disease for many years.

Polio, caused by a virus transmitted via contaminated food or water, was confirmed among young children in northeast Syria last month — its first appearance there in 14 years.

A health worker administers polio vaccine to a child as part of a UNICEF-supported vaccination campaign in Damascus, Syria last month. The oral form of polio vaccination is considered more reliable when the virus is actively circulating. (Omar Sanadiki/UNICEF/Associated Press)

The World Health Organization (WHO) said the virus probably spread from Pakistan — one of three countries where polio is still endemic — and warned that Syria's outbreak posed a threat to millions of children across the Middle East.

Polio passes easily from person to person and can spread rapidly among children, especially in the kind of unsanitary conditions endured by displaced people in Syria or in crowded refugee camps in neighbouring countries.

The disease invades the nervous system and can cause irreversible paralysis within hours — and the WHO's repeated warning is that as long as any single child remains infected with polio, children everywhere are at risk.

Vaccinating only Syrian refugees — as has been recommended by the European Centre for Disease Prevention and Control — must be judged as insufficient. - Martin Eichner and Stefan Brockmann

In their Lancet paper, Martin Eichner of the University of Tübingen and Stefan Brockmann of Germany's Reutlingen Regional Public Health Office noted that most European countries currently use inactivated polio vaccine (IPV) rather than oral polio vaccination (OPV) — a live form of immunization.

And while IPV is highly effective in preventing polio disease, it gives only partial protection from infection and is therefore less reliable if the virus is actively circulating.

Since large numbers of refugees are fleeing Syria and seeking refuge in neighbouring countries and Europe, there is now a chance the virus could be reintroduced into areas which have been polio-free for decades, they said.

"Vaccinating only Syrian refugees — as has been recommended by the European Centre for Disease Prevention and Control — must be judged as insufficient," they wrote. "More comprehensive measures should be taken into consideration."

Among extra measures, they suggested routine screening of sewage in areas where large numbers of Syrian refugees are settling, to check for the possible presence of polio virus.

Benjamin Neuman, a virologist at Britain's University of Reading, agreed that the fresh outbreak in Syria posed a wide risk both to neighbouring countries and beyond.

"Each new baby who is born is at risk of polio until vaccinated," he said. "Until the virus is completely extinct, it is essential that we continue to vaccinate our children."