Results from an early Ebola vaccine trial are said to be a "tad disappointing" but show that the drug is safe.
The trial conducted at Oxford University paves the way for the vaccine, jointly developed by GlaxoSmithKline (GSK) and the US National Institutes of Health (NIH), to be further tested on patients in West Africa.
The vaccine uses a single Ebola gene carried in a harmless chimpanzee cold virus to trigger an immune response.
A total of 60 healthy volunteers were inoculated at Oxford University's Jenner Institute between September 17 and November 18 last year.
Findings published in New England Journal of Medicine show the experimental vaccine was well tolerated in three different doses.
Two volunteers experienced a moderate fever which passed within a day.
Although the primary goal of the trial was to assess safety, scientists also measured immune responses to Ebola seen in the participants.
The vaccine was found to generate immunity with levels of antibodies increasing over a period of 28 days.
But independent experts said it did not perform as well as they would have liked.
Molecular virologist Professor Jonathan Ball, from the University of Nottingham, said: " The data have confirmed that the GSK vaccine is well tolerated, with only a small number of people experiencing very minor side effects, which is good news.
"The vaccine was also capable of provoking an immune response, although the overall potency of this response was a tad disappointing.
"The magnitude of the immune responses observed in the vaccine recipients were less than those observed in monkeys that were protected from experimental Ebolavirus infection, but we don't fully understand what degree of immunity humans require in order to protect them from infection.
"That's why clinical trials are important."
Dr Ben Neuman, a lecturer in virology at the University of Reading, said: " The good news is that the vaccine was safe, and nearly everyone who got it made some kind of new immune response.
"However, the response was smaller than I had hoped.
"These results show that the vaccine has the potential to work, particularly in the people who responded strongly, but I have some doubts about its ultimate effectiveness as the vaccine moves into tests in Africa."
The antibody response seen was weaker than that from a US study which tested a significantly higher dose of the same vaccine.
Generation of T-cells, another arm of the immune system, was also several times lower than that seen in immunised monkeys.
Trial leader Professor Adrian Hill, from the Jenner Institute, said: " The results are very encouraging in terms of the safety profile of the vaccine.
"That is the main outcome from this trial.
"We have seen an immune response in the great majority of people receiving the vaccine.
"It is possible to be optimistic about the immune responses we've seen; it's also hard to be really confident the levels would be protective.
"Larger trials in West Africa will be able to tell us more.
"We are also currently assessing another option, involving a booster dose, for improving immune response levels."
The Oxford University scientists have also started testing the safety of a candidate Ebola vaccine booster designed to increase the immune responses.
Thirty volunteers have been invited back to receive a second candidate Ebola vaccine of a different type made by Danish company Bavarian Nordic.
Anyone interested in volunteering for this trial should contact firstname.lastname@example.org.
Prof Hill added: "The speed at which all this is happening is remarkable.
"We'd especially like to thank all the volunteers.
"They continue to take time out of their busy lives to give blood samples at regular intervals so we can understand more about their immune responses having received the vaccine.
"It's also thanks to the hard work of many scientists, funders, pharma firms, regulators and agencies, all coming together, that we can make such rapid progress.
"Whether we have a vaccine that is safe, effective and works, we won't know for a while yet.
"But we owe it to the people who have been affected so badly by the Ebola outbreak to find out."
International Development Secretary Justine Greening said: "Britain is a world leader in medical research and the outcome of these DFID-funded trials takes us a step closer to finding a vaccine that could help contain Ebola and prevent future outbreaks.
"I would like to thank the volunteers who took part in this important trial.
"They join thousands of British doctors, nurses, military personnel, scientists and aid workers in the fight against this terrible disease."
Dr Jeremy Farrar, director of the Wellcome Trust, which co-funded the trial, said: "This study is very encouraging as it provides good initial evidence that the GSK vaccine will be safe to use in people.
"However, we still don't know whether it will provide protection against Ebola infection in a real-world situation.
"That's why trials in West Africa of this, and the other vaccines in development, must begin as soon possible.
"This has been an amazing global collaboration which we are proud to have supported.
"Falling infection rates in many of the affected regions increase the urgency of this research.
"While it is wonderful news that the epidemic is slowing in all three affected countries, we cannot consider the crisis over until there are no more people infected by Ebola across the whole region of West Africa.
"Vaccines may yet play a crucial role in achieving this, and will undoubtedly play a crucial role in protecting against future outbreaks of the virus."
Several trials of the GSK/NIH vaccine have been fast-tracked in the US, UK, Mali and Switzerland in response to the Ebola outbreak.
Since the start of the outbreak in March last year, Ebola has claimed more than 8,600 lives in Guinea, Liberia and Sierra Leone, Nigeria, Mali and the US.
More than 21,797 cases of the infection have been reported.