The first batch of GlaxoSmithKline's experimental Ebola vaccine has been dispatched to West Africa and is expected to arrive in Liberia later on Friday, the British drugmaker said.

The World Health Organization said on Thursday the Ebola outbreak in West Africa, the worst in history, appears to be waning but cautioned against complacency. The epidemic has seen 21,724 cases reported in nine countries since it started in Guinea a year ago. Some 8,641 people have died.

The shipment of an initial 300 vials of GSK vaccine will be the first to arrive in one of the three main Ebola-affected African countries, the company said in a statement.

It will be used in the first large-scale vaccine trials in coming weeks, in which healthcare workers helping to care for Ebola patients will be among the first to get it.

Researchers hope eventually to enrol up to 30,000 people in the trial, a third of whom would get GSK's candidate vaccine.

The vaccine, co-developed by the National Institutes of Health in the United States and Okairos, a biotechnology firm acquired by GSK in 2013, is now being tested in phase I safety trials in Britain, the United States, Switzerland and Mali involving around 200 healthy volunteers in total.

"Initial phase I data...are encouraging and give us confidence to progress to the next phases...which will involve the vaccination of thousands of volunteers, including frontline healthcare workers," said Moncef Slaoui, GSK's Global Vaccines chief.

The vaccine uses a type of chimpanzee cold virus to deliver safe genetic material from the Zaire strain of Ebola, the strain responsible for the unprecedented West African epidemic.

Data show the vaccine is safe in people, including in a West African population and in a range of dose levels, GSK said. It has now chosen the most appropriate dose for the Liberia trial.

Slaoui stressed that GSK's shot, like other candidates from a NewLink Genetics and Merck collaboration, and from Johnson & Johnson and Bavarian Nordic, remains in development and cannot be deployed unless and until it proves safe and effective.

Commenting on progress against the outbreak and on developing vaccines, Jeremy Farrar, director of Britain's Wellcome Trust health charity, said:

"This is certainly not the time for...efforts to be reduced. There is no doubt that we need vaccines and therapeutics for this epidemic and to try to prevent and respond to the inevitable future epidemics."