U.S. regulators have approved a vaccine to protect against the deadly Ebola virus, a crucial step in the battle against a virus that is considered one of the world's deadliest killers.

The vaccine, developed and made by Kenilworth, New Jersey-based Merck & Co., was found to be 100% effective when given at least 10 days in advance, the U.S. Food and Drug Administration said. The tests were conducted in Guinea, during a 2014 to 2016 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, in a statement.

The vaccine is given as a single injection, and 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, and the effectiveness of some inoculations can fade over time.

Drugmaker Johnson & Johnson has a second vaccine in development, though earlier this year public health authorities had made the Merck shot their focus. An ongoing outbreak in the Democratic Republic of Congo has killed more than 2,000 people, according to the World Health Organization.

In the U.S., ebola has typically been brought back by patients infected elsewhere who have in some rare circumstances infected the nurses and doctors caring for them. But the approval by the FDA marks a milestone in the fight against a terrifying virus that has killed thousands elsewhere.

"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 the communities facing or near an outbreak. Viruses depend on their hosts to spread, and vaccines -- whether for the flu, polio or other diseases — have been crucial tools in stopping some of the world's most dire illnesses.

Merck said in a statement that it was working on ramping up production of the vaccine to be available in the third quarter of 2020. In the meantime, it will continue providing a version of the shot that was used in the trial.

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