The World Health Organization (WHO) and news reports are describing the deployment of a new experimental vaccine for Ebola in the Democratic Republic of the Congo (DRC). Originally 4,000 doses were sent to the country, and while the number is growing to 8,000 or more, there are still not enough to widely inoculate the city of Mbandaka with a population of over a million. Reports describe how the vaccine will be used in a "ring vaccination" technique. In ring vaccination, those who are most likely to be infected receive the vaccine. Currently this is being done by inoculating the known contacts of the sick and the contacts of the contacts, as well as healthcare workers. Prior experiments suggest that the vaccine can prevent the disease in those individuals.
Yaneer Bar-Yam, Will the new ring vaccination stop the spread of Ebola?, New England Complex Systems Institute (May 23, 2018).