A new study has revealed that a novel vaccine temporarily shielded lab monkeys from Ebola, while a booster jab provided lasting protection.
A single shot of the vaccine, based on a cold virus that affects chimps, gave macaques "complete short-term and partial long-term protection" from the deadly virus, researchers reported in the journal Nature Medicine.
Animals that received a booster shot developed "durable" immunity.
The US National Institute of Allergy and Infectious Diseases (NIAID) announced on August 28 that human testing would start in early September following the promising results in monkeys.
The results of these early trials are expected by year's end.
If approved, "this vaccine will be beneficial for populations at acute risk during natural outbreaks or others with a potential risk of occupational exposure," said the study's authors.
They claimed to be the first to demonstrate a vaccine with "durable protection" against Zaire ebolavirus, which has killed 1,841 of the 3,685 people infected in Guinea, Liberia and Sierra Leone, according to World Health Organisation (WHO) numbers.
Nigeria has had another 21 cases and seven deaths from the haemorrhagic fever against which no licenced vaccine or treatment exists.
Nancy Sullivan of NIAID's Vaccine Research Center led a team that developed the vaccine based on a type of chimpanzee virus dubbed ChAd3.
The virus is used as a carrier, or vector, to deliver snippets of Ebola DNA into human cells.
The genetic material is non-infectious, but stimulate the cells of the vaccine recipient to recognise Ebola and mount an immune response against it.
The carrier virus is from the adenovirus family, which can cause colds, diarrhoea or bladder infection, and does not replicate in the vaccine recipient.
The research team said they had opted for a chimp adenovirus rather than a human one because many people have been exposed and developed immunity to the human versions, meaning their immune systems would be primed to neutralise them.
For the study, the team gave different doses of the ChAd3 vaccine to a small group of monkeys, and then a dose of Ebola that would be lethal in non-immune individuals.
Some of the macaques also received a booster vaccine.
Four monkeys that received a single shot were immune when exposed to Ebola five weeks later, the researchers reported.
The protective effects waned over time, and by 10 months only two of the monkeys were still protected.
All four monkeys that were given the initial shot and a booster eight weeks later were fully protected against infection ten months after the initial injection, said the team.
The booster shot used a different vector, a pox virus, to deliver the Ebola genetic material.
The early trials in humans are the first phase in a three-step vetting process.
They will be carried out among healthy adults not infected with the Ebola virus to determine if it is safe and induces an adequate immune response.