Health experts have warned that the recent outbreak of polio in Syria poses a serious threat to Europe, where the disease was eradicated in 2002.
Europe is exposed because some countries have low rates of innoculation, which lowers "herd immunity", or protection through community-wide vaccination, a pair of German epidemiologists warned in The Lancet.
In addition, most countries use a type called inactivated polio vaccine (IPV), which is less effective against the virus than an oral vaccine which is now largely discontinued because it causes rare cases of paralysis, they warned.
At least 10 cases of polio have been confirmed by the World Health Organisation (WHO) in Syria, where vaccination has been disrupted by the war.
The polio virus has also been found in Israel in routine tests in sewage, from people who had the virus but did not develop the disease, prompting the country to launch an emergency innoculation campaign.
In their letter to The Lancet, Martin Eichner of the University of Tuebingen and Stefan Brockmann, a regional health officer in Reutlingen, said the risk of polio spreading to Europe was invisible but real.
Out of every 200 people infected with the polio virus, only one will develop the disease's classic symptoms, called acute flaccid paralysis, they said.
"In regions with low vaccination coverage (e.g. Bosnia and Herzegovina, 87 percent or Ukraine, 74 percent), particularly those with low coverage of inactivated polio vaccine (e.g. Austria, 83 percent), herd immunity might be insufficient to prevent sustained transmission," said the letter.
In addition to vaccinating Syrian refugees, "more comprehensive measures" should be considered, it said.
"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."