The Syrian government excluded the largely rebel-held province of Deir al-Zor from a 2012 polio vaccination campaign because, it claimed, most residents had fled the region during the conflict.
The vaccination campaign was targeted at two million children in 13 out of 14 of Syria's governorates.
However, according to a Reuters investigation, thousands of Syrians were still in the region during the vaccination.
Syria eliminated polio in the 1990s but the disease has re-emerged because of the war and because it was not eradicated in Deir al-Zor.
The World Health Organisation (WHO) reported that a cluster of suspected polio cases was detected in early October in the province.
Initial results from a laboratory in Damascus indicated that at least two cases could be polio.