The first large-scale trial of an Ebola vaccine will begin in Guinea at the weekend, said the World Health Organization, weeks after a similar test kicked off in neighbouring Liberia.
The Phase III testing of the vaccine -- one of two that are in the most advanced stages of development -- aims to ensure it provides protection against the virus that has killed some 9,800 people in Liberia, Guinea and Sierra Leone.
There is no licensed treatment or vaccine for Ebola, and the WHO has endorsed rushing potential ones through trials in a bid to stem the epidemic.
Researchers have said that it remains unknown what level of immune response is needed to protect humans from Ebola, which causes often fatal haemorrhaging, organ failure and severe diarrhoea.
"If a vaccine is found (to be) effective, it will be the first preventive tool against Ebola in history," WHO chief Margaret Chan said in a statement.
The two candidate vaccines currently being developed -- CAd3 by Britain's GlaxoSmithKline and VSV-EBOV by Merck and NewLink Genetics -- have both passed safety tests on humans.
They are already being tested in Liberia, and the trial of VSV-EBOV will kick off in Guinea on March 7, WHO said.
Vaccinations will take place in areas of Basse Guinee, the region that currently has the highest number of cases in the country, it added.
Authorities will use a so-called "ring vaccination" approach that was used to eradicate smallpox in the 1970s, tracking down anyone who could have been exposed to a newly diagnosed patient and vaccinating them if they agree.
Part of the contacts will be vaccinated immediately after the new case is detected, while others will be given a shot three weeks later -- the length of the virus's incubation period.
"The Ebola epidemic shows signs of receding," said Marie-Paule Kieny, who leads the Ebola research and development effort at WHO.
In worst-hit Liberia, for instance, no new confirmed cases have been reported for a full week, WHO said, a first since May.
"But we cannot let down our guard until we reach zero cases," Kieny said in the statement.
"An effective vaccine to control current flare-ups could be the game-changer to finally end this epidemic and an insurance policy for any future ones."