Image caption Oral polio vaccine is given in two drops

Europe could be at risk from polio following a recent outbreak in Syria, infectious disease experts say.

In the Lancet journal, two doctors in Germany say the cases in Syria - which had been free of wild poliovirus since 1999 - could endanger nearby regions.

They say because only one in 200 people infected develops paralysis it could take a year of "silent transmission" before an outbreak is detected.

In that time hundreds of individuals could be carrying the infection.

Prof Martin Eichner, of the University of Tubingen and Stefan Brockmann, of Reutlingen Regional Public Health Office explain that most European countries use inactivated polio vaccine (IPV) rather than the live oral polio vaccine (OPV), because the latter can, in rare cases, lead to cases of acute flaccid paralysis, the main symptom of polio.

Threat

Whilst IPV is highly effective at preventing polio disease, it does not give the same level of protection against the virus as the oral drops, so vaccination coverage needs to be very high. The doctors say that countries with low coverage such as Austria (83%) and Ukraine (74%) risk a sustained outbreak should the virus be introduced via refugees fleeing Syria. Polio vaccine coverage in the UK is at 95%.

The doctors said Israel could also pose a potential polio threat.

Prof Eichner told me: "Wild poliovirus has also been found in sewage in Israel and from samples taken from some symptom-free individuals since February 2013. Although there have been no cases of polio in Israel, tourists could risk bringing the infection to other countries."

Dr Benjamin Neuman, a virologist at the University of Reading, said: "The Syrian outbreak puts Europe at risk because of the way we give vaccines. In parts of the world where it is still possible to catch a wild strain of poliovirus, children are usually vaccinated with a live but genetically weakened poliovirus which gives excellent protection but has a tiny risk of changing back to the more dangerous form.

"However, in parts of the world where polio has been eradicated, like the UK, children are usually given a killed vaccine. It doesn't protect quite as well but it cannot mutate, so it protects reasonably well while preventing polio from being accidentally reintroduced to a country.

He added: "Vaccination is never perfect, so despite being vaccinated, a small percentage of children in the UK would be at risk of contracting polio if they were exposed to the virus. Until the virus is completely extinct, it is essential that we continue to vaccinate our children."

The Department of Health Director of Immunisation, Professor David Salisbury said:

"The UK is well defended against the possibility of importation of polio from outbreaks such as the present one in Syria. Our population has very high coverage of polio vaccination with more than 95% of young children being vaccinated. We have WHO-approved surveillance in place so we can pick up polio if it does start to circulate in the UK."

'Invisible enemy'

Most of the 22 cases of polio-like paralysis in Syria (10 of which have been confirmed as wild poliovirus type) are among children below the age of two who were unimmunised or who had not received all three doses of vaccine.

The World Health Organization warned last month there was a high risk of polio spreading internationally because of the movement of refugees across the region and low immunisation rates in Syria.

Prof Eichner said: "The WHO wants to get rid of polio completely and had got pretty close until recent outbreaks. The fact that most of those infected do not display symptoms but can still spread the disease makes it a very hard virus to get rid of as it is like fighting an invisible enemy."

So far this year 322 cases of wild poliovirus have been confirmed, more than half of them in Somalia.