Sudan is preparing a mass vaccination campaign against polio after an outbreak, the UN said Friday, days after the declaration of the eradication of wild poliovirus in Africa.

On Tuesday, the World Health Organization (WHO) celebrated the announcement that Africa was free of cases of the wild virus that causes the crippling disease, a landmark in a decades-long campaign to eradicate it.

The 13 cases in Sudan are derived from a vaccine, which can take place in rare cases when the weakened virus in the vaccine mutates, a problem in countries with low immunisation rates and poor sanitation, health experts say.

"This is the first polio outbreak in Sudan in more than a decade, and has affected nine states across the country," the UN's Office for the Coordination of Humanitarian Affairs (OCHA) in Sudan said.

The first case was recorded on August 9, and areas affected include the war-ravaged western region of Darfur, it said.

"At least 13 cases of circulating vaccine-derived poliovirus have been confirmed," it added.

The Sudanese health ministry, alongside UN agencies, is planning to vaccinate five million children aged under five.

Vaccination programmes like this one in Nigeria in 2017 eradicated wild poliovirus in Africa but vaccine-derived polio still poses a threat in many countries
Vaccination programmes like this one in Nigeria in 2017 eradicated wild poliovirus in Africa but vaccine-derived polio still poses a threat in many countries AFP / PIUS UTOMI EKPEI

"Although Africa was recently declared free of the wild poliovirus... vaccine-derived polio persists and poses a threat to many countries," the statement said.

"Neighbouring Ethiopia, Central African Republic and Chad also reported cases, which puts more risks on Sudan, especially in the Darfur region, due to border movements."

An oral vaccine given to young children has been massively effective against polio around the world.

Vaccine-derived outbreaks are stopped by using the same health tactics as eradicated wild polio.

That includes the intense monitoring of populations and ensuring high vaccination coverage.

Poliomyelitis -- the medical term for polio -- is an acutely infectious and contagious virus which attacks the spinal cord and causes irreversible paralysis in children.

It is typically spread in the faeces of an infected person and is picked up through contaminated water or food.

Sudan Central African Republic Ethiopia