Close to 12 million people in Benin, Liberia and Sierra Leone will be vaccinated against yellow fever in the largest health campaign in Africa, the World Health Organization (WHO) said Tuesday.

"High vaccination coverage will prevent outbreaks of yellow fever, a disease that is very difficult to diagnose in the early stages of infection," said William Perea, coordinator of WHO's epidemic readiness and intervention unit.

Recent yellow fever outbreaks in these three African nations have been severe.

"A single dose of the vaccine offers full protection." Perea said.

A total of 29 million people have been vaccinated against yellow fever in Burkina Faso, Cameroon, Mali, Senegal and Togo since 2007, WHO said.

But the agency cautioned that more funding is needed to protect another 160 million people who could still be at risk in Africa.

"Yellow fever is reappearing in countries that have not reported cases in many years," said Fenella Avokey, medical officer for WHO's yellow fever control regional office.

WHO is to be joined in this effort by the UN Children's Fund, national Red Cross and Red Crescent Societies, Doctors Without Borders and other partners.

Sapa