Yellow fever can cause devastating outbreaks in tropical areas of South America and Africa. In the spring of 2016 an outbreak occurred in Angola and the Democratic Republic of the Congo, Kenya, and Mauritania. It has spread from there to China where it could be devastating if an outbreak occurs. Because yellow fever is spread by the same mosquito as the dengue virus, there are 300 million people at risk in the dengue-endemic provinces. Vaccine stores have been scarce from use in Africa and it takes time to replenish them.