The U.S. Food and Drug Administration approved Merck & Co. Inc.'s Ervebo, making it the first approved vaccine in the U.S. for Ebola virus disease.

Ebola virus disease, or EVD, is contagious and transmitted through direct contact with blood, body fluids and tissues of infected wild animals or people, as well as with surfaces and materials contaminated with the fluids. Cases of the disease are very rare in the U.S, the FDA said in a news release.

Ervebo is a single-dose injection, and is a live, attenuated vaccine — which means the virulence of the pathogen has been reduced — that has been genetically engineered to contain a protein from the Zaire ebolavirus.

The approval is supported by a study conducted in Guinea in individuals 18 years or older during the outbreak between 2014 and 2016. The study involved individuals with laboratory-confirmed EVD who received either "immediate" or 21-day "delayed" vaccination with Ervebo.

During the trial, a comparison of 2,108 individuals in the "immediate" vaccination arm and 1,429 individuals in the "delayed" vaccination arm showed Ervebo was 100% effective in preventing Ebola cases with the onset of symptoms more than 10 days after vaccination.

Additionally, Ervebo was assessed in 477 individuals in Liberia, about 500 individuals in Sierra Leone and about 900 individuals in Canada, Spain and the U.S. Antibody responses among those in the study conducted in Canada, Spain and the U.S. were similar to those seen in trials conducted in Liberia and Sierra Leone.

Merck & Co.'s application for Ervebo was previously granted priority review and tropical disease priority review voucher designations by the U.S. FDA. The vaccine was also granted breakthrough therapy designation to facilitate the development and evaluation of the vaccine.

Ervebo was also granted conditional approval by the European Commission in November under an accelerated assessment process.