virus, raising a prospect of successful human trials, say scientists.
The experiments by the US National Institutes of Health showed
immunity could last at least 10 months.
Human trials of the vaccine started this week in the US and will
extend to the UK and Africa.
The World Health Organization says more than 2,000 people have now
died in the outbreak in West Africa.
Several experimental treatments are now being considered to help
contain the spread of Ebola. This includes a vaccine being developed
by the US National Institute of Allergy and Infectious Diseases and
pharmaceutical company, GlaxoSmithKline.
It uses a genetically modified chimp virus containing components of
two species of Ebola - Zaire, which is currently circulating in West
Africa, and the common Sudan species.
The viral vaccine does not replicate inside the body, but it is hoped
the immune system will react to the Ebola component of the vaccine and
develop immunity.
Poor durability
Animal research, on which the decision to begin human trials was
based, has now been published in the journal Nature Medicine.
It shows four crab-eating macaques all survived what would have been a
fatal dose of Ebola virus five weeks later.
However, only half survived an infection 10 months after immunisation.
Dr Anthony Fauci, the Director of the US National Institute of Allergy
and Infectious Diseases, told the BBC: "The good part of this vaccine
is that at five weeks or earlier you get full protection.
"The sobering news is the durability isn't great, but if you give a
boost, a second shot, you make it really durable.
"We knew this worked in the monkey months ago and based on this paper
we started human trials."
For now this is the best evidence available on how successful such a
vaccine would be in people.
The first patient, a 39-year-old woman, was given the vaccine last
week as human trials got under way.
There will also be separate trials of the vaccine against just the
Zaire Ebola species. These will take place in the US, the University
of Oxford in the UK as well as in Mali and Gambia.
The WHO said safety data would be ready by November 2014 and, if the
vaccine proved safe, it would be used in West Africa immediately.
Healthcare workers and other frontline staff would be prioritised for
vaccination.
'Really encouraging'
The number of doses currently available is between 400 - if a lot of
vaccine is needed for immunity - and 4,000 if smaller amounts are
sufficient.
As with all experimental therapies, the WHO has warned hopes of a
vaccine must not detract from the proven methods of infection control
which have defeated all previous outbreaks.
Prof Jonathan Ball, a virologist at the University of Nottingham,
said: "This is really encouraging data.
"The degree of protection seen with the chimpanzee adenovirus alone -
which will be used in one of the human clinical trials planned for the
UK, Mali and the Gambia - was still pretty impressive, especially when
the animals received Ebola virus within a few weeks of vaccination.
"This is important as it would keep the dosing regimen simple and
could still provide good protection in the sort of outbreak that we
are seeing in Western Africa at the moment."
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