The world’s largest mass yellow fever immunization is starting in West Africa, targeting 12 million people, writes Times Live. World Health Organization will begin vaccinating in Benin, Liberia and Sierra Leone. WHO spokeswoman, Aphaluck Bhatiasevi indicated that local health teams will also offer vitamin A and deworming tablets and the measles vaccine in Sierra Leone. Thirteen African nations are said to be at high risk of outbreaks of yellow fever, which can kill if untreated.