Health workers in the Democratic Republic of Congo are to begin an immunization campaign in an attempt to halt the spread of the deadly Ebola virus.
The experimental vaccine proved effective when used in limited trials during the epidemic which struck West Africa in 2014-16.
At least 25 people are believed to have died in the current outbreak.
Health workers will be among the first to receive the vaccine on Monday.
The vaccine, made by pharmaceutical firm Merck, is not yet licensed, but was effective in limited trials during the West Africa outbreak.
Dr Michel Yao, from the WHO, told the BBC that the vaccine had been tested in Guinea and that "almost all of the people who were vaccinated could not get the disease".
The WHO has sent more than 4,000 doses to the Democratic Republic of Congo, with another batch set to follow.
Health care providers and funeral workers will be vaccinated initially, before the programme is extended to more than 500 people who may have come into contact with those infected with the virus, in a so-called "ring vaccination".
At least 45 cases of Ebola have been reported, including three health workers, since the outbreak began earlier this month.
The virus has already spread from rural areas to the north-western city of Mbandaka, a major transport hub on the River Congo, where at least four cases have been confirmed.
This has sparked fears that the outbreak could reach the capital, Kinshasa, as well as neighbouring countries.
However, the World Health Organization (WHO) has said it has "strong reason to believe that the outbreak can be brought under control".
At an emergency meeting, on Friday WHO experts said that "the conditions for a Public Health Emergency of International Concern (PHEIC) have not currently been met".
This is the ninth outbreak of Ebola in DR Congo - it was named after the country's Ebola river.
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