Sudan has launched a three-day campaign in the north of the country to immunise an estimated five million children against polio after reports of cases in neighbouring Chad.

The campaign, led by the federal health ministry, supported by the UN Children’s Fund (UNICEF) and the World Health Organization (WHO), entered its second day on 7 August as health officials carried out door-to-door innoculations.

The immunisation drive was expected to cover all 15 states in the north of Sudan and organisers hoped to reach at least 82 percent of children under the age of five during the three-day effort.

Officials mobilised tens of thousands of vaccinators but admitted they may not be able to reach children in areas made inaccessible by heavy rains and floods that have been battering much of northern Sudan over the past weeks.

"There has not been a single case of polio in Sudan since June 2005, and we have arrived – in the face of many challenges – at a point where polio could soon be stamped out in Sudan," according to UNICEF representative, Ted Chaiban, in a statement released on the eve of the campaign.

"But because polio respects no borders, we have to ensure that when cases are found close to home, we redouble our efforts to protect children," he added.

Sudan has a long and open border with Chad in the troubled western region of Darfur, where 200,000 people have died and more than two million been driven from their homes in more than four years of conflict between rebels and the government.

Sudan’s federal health ministry and the WHO established an active and sensitive Acute Flaccid Paralysis surveillance system in 2000 to monitor potential outbreaks.

"In order to eradicate polio, it is necessary to search out not only the polio cases, but also conditions that may clinically resemble polio," said WHO representative Mohamed Aburrab.

Sudan has over the past years organised regular polio campaigns across the country, including in conflict-affected areas such as Darfur. UNICEF, the WHO and other agencies have been calling for unhindered access for vaccinators.

However, "increased population displacement, porous borders with neighbouring countries and continued insecurity" remain a threat to vaccination efforts in the region, according to UNICEF.

The agency said the three-day effort in the relatively peaceful north also faced some challenges. "Recent heavy rains and flooding in many parts of north Sudan may lead to gaps in the upcoming campaign, resulting in some children being missed in affected areas," it stated. "Plans are being developed to reach such children using all available means at the first opportunity that allows access to missed areas."

The last three-day vaccination campaign took place at the end of April this year when vaccinators managed to reach more than six million children.

UNICEF has provided vaccine and training for vaccinators in Sudan’s regular polio campaigns, with the WHO paying half the operational costs.

Vaccinators are sourced through local state health ministries, non-governmental organisations and from among local communities. Donors include the US Centers for Disease Control, USAID, Rotary International, the World Bank, and the Bill and Melinda Gates Foundation.