The Syrian government excluded the largely rebel-held province of Deir al-Zor - where polio broke out this year - from a 2012 vaccination campaign, arguing that most residents had fled although hundreds of thousands were still there, a Reuters investigation shows.

Public health researchers say missing out the Syrian province contributed to the reemergence there of polio, a highly infectious, incurable disease that can paralyze a child within hours but has been wiped out in many parts of the world.

In November, the World Health Organisation (WHO) said 13 cases had been found in the province. Two more have since been recorded there and the virus has surfaced in Aleppo city and near Damascus, the first outbreak since 1999 in Syria, where civil war has raged since a crackdown on protests in 2011.

A Dec 6, 2012, WHO statement said it, in conjunction with the Syrian Ministry of Health and the United Nations Children's Fund, had launched a campaign to vaccinate "all children below the age of five against polio".

It said the campaign, involving 4,000 health workers and volunteers, would cover roughly 2.5 million children in 13 of Syria's 14 governorates except for Deir al-Zor as "the majority of its residents have relocated to other areas in the country".

It was not possible to contact the Syrian government for comment on its reported decision to leave out Deir al-Zor, a region of roughly 1.2 million people, where more than 600,000 under 15 years were living in 2012, according to WHO data.

By December of that year, rebels had taken territory in other provinces as well.

While international agencies support such vaccination campaigns, designed to fill gaps left when emergencies prevent routine vaccinations, it is a country's government which decides when and where they will take place.

Asked to comment on researchers' allegations aid groups should have raised the alarm earlier and prepared better, Chris Maher, who is coordinating the regional polio response for the WHO, said it had warned vaccination rates were falling.

The December 2012 and the Oct-Nov 2013 campaigns were planned and organized in response to that, he said. "In a complex emergency setting, it is not that easy to continue routine campaigns."

Maher said it was reported that 67,000 children under the age of five were subsequently vaccinated in Deir al-Zor in January 2013.

Public health researchers say that is a coverage rate of around 50 per cent, insufficient to prevent polio from spreading, based on census data. The actual population is hard to establish; some residents fled while other people fled into Deir al-Zor from elsewhere.

Repeated vaccinations and high coverage levels are needed to interrupt transmission of the virus and prevent outbreaks.

"There was a lack of a proper campaign to vaccinate children across the country over the past two years," said Dr Adam Coutts, a Lebanon-based public health researcher who has been studying the humanitarian response in Syria.

"With the breakdown of the health system, sanitation and nutrition, the exclusion of Deir al-Zour from the vaccination campaign provided the ideal conditions for an outbreak to occur."

It was not clear why the remote province near Syria's border with Iraq was singled out. The city of Deir al-Zor is partially controlled by Syrian government forces while the countryside around it is in the hands of rebels fighting to remove President Bashar al-Assad.

Maher did not say whether there were other vaccination campaigns in Deir al-Zor during 2012 but confirmed that there was one in October this year, around the same time that polio cases were found in Deir al-Zor.

Asked if he thought leaving a gap in the 2012 campaign allowed polio to take hold in Deir al Zor, Maher said: "There are unimmunized kids all over Syria."

"I have no information that that particular area was higher risk than anywhere else given the general deterioration of immunization rates during the conflict."

He said polio vaccination coverage had dropped across Syria from more than 90 per cent in 2010 to below 70 per cent in 2012.

United Nations humanitarian agencies work in Syria with the permission of the Syrian government, which has blocked aid convoys to some areas of the country. Opposition fighters and clashes have also hampered access for aid work.

Despite dramatic progress many parts of the world thanks to a 25-year-old campaign to eradicate the disease, Polio is still endemic in Pakistan, Afghanistan and Nigeria.

There is no cure and it can only be prevented through immunization, usually three doses. The WHO's long-standing and repeated warning on the disease is that as long as any child remains infected, children everywhere are at risk.

"Questions remain as to why WHO did not better prepare for this, given their own recognition about the risk of outbreaks," said Coutts, whose colleague Dr Fouad Fouad shares his concern.

The WHO says the largest-ever immunization response in the Middle East is under way, aiming to vaccinate more than 23 million children against polio in Syria and neighboring countries.

"Inside Syria, the campaign aims to reach 2.2 million children, including those who live in contested areas and those who were missed in an earlier campaign. Many children in Syria remain inaccessible, particularly those trapped in sealed off areas or living in areas where conflict is ongoing," it said.

The WHO says almost 2 million children in Syria have already been vaccinated, including 600,000 in contested areas of the country, in the first of several rounds.

Coutts says public health professionals in the region are concerned that this response is "too little too late and is exposing a deeper failure of regional health agencies and systems to respond to a very predictable health crisis".-Reuters