U.S. regulators have approved a vaccine to protect against the deadly Ebola virus, one of the world’s deadliest killers and one that caused a public health scare in Dallas in 2014.

The vaccine, developed and made by Merck & Co., was found to be 100% effective when given at least 10 days in advance, the Food and Drug Administration said. The tests were conducted in Guinea during a 2014-16 outbreak.

“The FDA’s approval of Ervebo is a major advance in helping to protect against the Zaire ebolavirus as well as advancing U.S. government preparedness efforts,” said Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research.

The vaccine, given as a single injection, appears to be most effective when patients have time to build immunity before being exposed to the virus. It’s not clear how long protection from the shot lasts.

An ongoing outbreak in Congo has killed more than 2,000 people, according to the World Health Organization.

In the United States, ebola has typically been brought back by people infected elsewhere who have, in rare circumstances, infected the nurses and doctors caring for them.

“While the risk of Ebola virus disease in the U.S. remains low, the U.S. government remains deeply committed to fighting devastating Ebola outbreaks in Africa, including the current outbreak in the Democratic Republic of the Congo,” said Anna Abram, FDA deputy commissioner for policy, legislation and international affairs.

Vaccines can be deployed to inoculate health workers or to build a wall of immunity in communities facing or near an outbreak.

Merck said it was working to ramp up production of the vaccine.