An end to the Ebola epidemic in West Africa may be in sight after results from a fast-tracked vaccine trial were described as "hugely important" and "very exciting" by experts.
The experimental vaccine VSV-ZEBOV was found to be 100% effective six days after it was administered to 7,651 people living in Guinea, where the virus has already claimed more than 2,500 lives.
It made no difference if people had the jab immediately or following a delay of three weeks after giving their consent to treatment.
Critically, non-vaccinated individuals were also protected. A "ring vaccination" strategy - used in the past to eradicate smallpox - was adopted to create a buffer zone of protection by reducing spread of the disease.
Commenting on the findings, published in The Lancet medical journal, Dr Jeremy Farrar, director of Britain's biggest research charity the Wellcome Trust, said: "Our hope is that this vaccine will now help bring this epidemic to an end and be available for the inevitable future Ebola epidemics.
"It should change how the world responds to emerging infectious disease threats."
The trial was given the title "Ebola ca Suffit", which means "Ebola this is Enough".
It took place in Basse-Guinea, the only district of Guinea with new Ebola cases at the start of the study on April 1 this year.
For every individual confirmed as being newly infected, researchers traced all the people he or she may have been in close contact with.
Adult contacts aged 18 and older who were not pregnant or breastfeeding were offered the vaccine. If consent was given, adults were randomised either to receive it immediately or after a delay.
Comparisons between an active and "dummy" placebo vaccine could not be made for ethical reasons, since it would have meant condemning one group of participants to a high risk of death.
Under the ring vaccination strategy, vaccination was followed up by monitoring the contacts and contacts of contacts of an immunised individual.
Dr Marie Paule Kieny, one of the researchers from the World Health Organisation in Geneva, Switzerland, said: "Before the trial started, in most (population) clusters there had been a series of Ebola cases over the weeks prior to randomisation. However, since the trial started, we have seen no new cases in vaccinated volunteers within 10 days of vaccination, regardless of whether vaccination was immediate or delayed."
In fact the vaccine appeared to be offering protection in as little as six days.
A total of 16 new cases were recorded in the "delayed" vaccination clusters, which included unvaccinated contacts.
Taking account of untreated as well as treated individuals, the vaccine provided 75% overall protection to communities participating in the trial.
"We don't know how long the effects of the vaccine last, but the best use for this vaccine may be in ring-fencing suspected Ebola cases by vaccinating the people who have had close contact with Ebola patients including their families and medical workers," Dr Kieny added.
VSV-ZEBOV was developed by the Public Health Agency of Canada and is licensed to NewLink Genetics and Merck.
The vaccine contains no live Ebola virus. It works by replacing a gene from a harmless virus known as vesicular stomatitis virus (VSV) with one encoding an Ebola virus surface protein. This is enough to prompt the immune system to launch a defensive response against Ebola.
Dr Dan O'Connor, head of humanities and social science at the Wellcome Trust, said: "The success of the 'ring' strategy is a hugely important step forward for research ethics. It shows not only that we can conduct urgently needed, rapid response research during epidemics, but that we can do so in a way that is both ethically sound and responsive to the needs of affected communities."
Professor Peter Smith, from the London School of Hygiene & Tropical Medicine, said: "The interim results of the trial, using a novel evaluation approach, are very exciting and suggest that the Ebola vaccine tested may be highly effective in protecting against Ebola disease among those in the immediate vicinity of an Ebola case.
"If these results are confirmed as the trial continues, the ring vaccination strategy may be important not only in contributing to ending the current epidemic, but will also be deployable to contain rapidly any future outbreaks of Ebola disease."
Dr Farrar called the immediate-versus-delayed vaccination programme a "highly innovative and pragmatic design" that made it possible to assess a vaccine in the middle of an epidemic.
He added: "It is a remarkable result, and one that shows the power of equitable international partnerships and flexibility."
Dr Ben Neuman, lecturer in virology at the University of Reading, said: "This is big news - the most promising medical development so far in the ongoing race to shut down Ebola."