GENEVA, Nov. 17 (Xinhua) -- The largest-ever yellow fever mass vaccination campaign is set to kick off next week across three African countries -- Benin, Liberia and Sierra Leone, the World Health Organization (WHO) said on Tuesday.

    The week-long event will target 11.9 million people across the three countries, all of which are at high risk of yellow fever outbreaks, the UN agency said in a statement.

    The campaign, supported by the WHO, the United Nations Children's Fund, Mdecins sans Frontires and other partners, is the first in which yellow fever vaccination drives will be simultaneously launched across several countries.

    "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 Epidemic Readiness and Intervention Unit.

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

    Benin, Liberia and Sierra Leone are the latest three of the 13 highest risk African countries to carry out preventive campaigns, the WHO said.

    Since 2007, a total of 29 million people have been protected through mass vaccinations conducted in Burkina Faso, Cameroon, Mali, Senegal and Togo, as well as a first phase completed in Sierra Leone.

    Perea expressed hope that vaccination campaigns would be carried out throughout all high risk African countries by 2015. Those countries also include the Central African Republic, Cote d'Ivoire, Ghana, Guinea and Nigeria.