The World Health Organisation announced on Tuesday that the largest ever round of yellow fever vaccinations, for about 12 million people, would be launched in three West African countries next week.

"The largest ever drive against yellow fever is going to kick off in three countries in West Africa next Monday," said WHO spokeswoman Aphaluck Bhatiasevi.

"These three countries are Benin, Liberia, Sierra Leone and 11.9 million people will be vaccinated in ten days," she told journalists.

The vaccination drive, which is also backed by the UN Children's Fund (UNICEF), the Red Cross and Doctors Without Borders (MSF), was launched in 13 West African countries at the end of 2007.

Some 29 million people have been vaccinated against yellow fever so far, according to the WHO.

About 30,000 people die every year from the viral disease, which is carried by mosquitoes. Its most virulent form can kill more than 50 percent of those infected.

The UN health agency has blamed deforestation, urban growth, climate change and low levels of immunisation for a resurgence of the disease in recent years, mainly in South America and in Africa.