A vaccine developed by Merck is the first shown to be highly effective in preventing human infection with Ebola, according to final results from a clinical trial.
Among 5,837 people who received the rVSV-EBOV vaccine in the trial in Guinea in 2015, no Ebola cases were recorded 10 days or more after vaccination, signaling 100 percent protection.
More than 11,300 people died in West Africa's 2013-2016 epidemic of the virus, which causes hemorrhagic fever.
"When the next Ebola outbreak hits, we will not be defenceless," said Marie-Paule Kieny, a World Health Organization (WHO) assistant director-general and one of the lead researchers in the trial.
Merck's rVSV-EBOV has already been used in Sierra Leone to contain a flare-up there.
The Guinea trial took place in the coastal region of Basse-Guinée, which was still seeing new Ebola cases when the trial started in 2015.
It used a "ring vaccination" approach, in which, when a new Ebola case was diagnosed, researchers traced all people who might have been in recent contact and logged them as clusters, or "rings", made up of an average of 80 people.
At first, the clusters were assigned to receive the vaccine either immediately or after a three-week delay. After interim results showed 100 percent protection in those vaccinated immediately, the trial design was switched to allow all clusters to be given the shot straight away.
The WHO said that if Ebola outbreaks occurred before the vaccine was given full regulatory approval, it could be made available though a compassionate use procedure after informed consent.
John Edmunds, a professor of infectious disease modeling at the London School of Hygiene & Tropical Medicine, whose team helped design the trial, hailed its success as "historic".
"When Ebola strikes again we will be in a much better position to offer help to affected communities, as well as protect the brave volunteers who help control this terrible disease," he said in an emailed statement.
GAVI, the global vaccine alliance, provided $5 million earlier this year to buy 300,000 doses of rVSV-EBOV as a stockpile for use during future Ebola outbreaks.