Vaccinated monkeys have developed “long-term” immunity to the Ebola 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.
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.
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.”
Ebola virus disease (EVD)
- Symptoms include high fever, bleeding and central nervous system damage
- Spread by body fluids, such as blood and saliva
- Fatality rate can reach 90% – but current outbreak has mortality rate of about 55%
- Incubation period is two to 21 days
- There is no proven vaccine or cure
- Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery
- Fruit bats, a delicacy for some West Africans, are considered to be virus’s natural host