The US Food and Drug Administration (FDA) has approved pharmaceutical company Merck's Ebola vaccine, which is being used to fight the deadly virus in the Democratic Republic of Congo.This makes it the first FDA-authorised vaccine against the deadly virus, reports Reuters news agency.The World Health Organization and the European Commission approved the vaccine earlier this year.Using unauthorised vaccines in the Ebola outbreak in eastern DR Congo has proved a controversial move.In August DR Congo's health minister said the Congolese population were being used as guinea pigs when a second vaccine, by Johnson & Johnson, was introduced. He later resigned.But the only way to test its effectiveness as prevention measure for humans is for it to be used in an outbreak scenario.This is how the Merck vaccine was successfully deployed in Guinea in 2015.Since then, more than 150,000 have been given the Merck vaccine in the DR Congo outbreak and it has proved 97% effective.