The vaccine prompted no serious side effects and elicited antibody and T-cell immune responses, according to trial results published in The Lancet medical journal.
An experimental vaccine being developed by AstraZeneca and Oxford University against the coronavirus produced an immune response in early-stage clinical trials, data showed on Monday, preserving hopes it could be in use by the end of the year.
The vaccine, called AZD1222, has been described by the World Health Organisation's chief scientist as the leading candidate in a global race to halt a pandemic that has killed more than 600,000 people.
More than 150 possible vaccines are in various stages of development, and US drugmaker Pfizer and China's CanSino Biologics also reported positive responses for their candidates on Monday.
The vaccine from AstraZeneca and Britain's University of Oxford prompted no serious side effects and elicited antibody and T-cell immune responses, according to trial results published in The Lancet medical journal, with the strongest response seen in people who received two doses.
Co-author Professor Sarah Gilbert from the University of Oxford said the results "hold promise".
"If our vaccine is effective, it is a promising option as these types of vaccine can be manufactured at large scale."
British Prime Minister Boris Johnson, whose government has helped fund the project, hailed the results as "very positive news" though the researchers cautioned the project was still at an early stage.
For its trial, the team at Oxford used a genetically modified strain of the common cold virus that infects chimpanzees.
They manipulated the virus to train cells to recognise the viral spike protein, which helps teach the immune system to recognise COVID-19.
As well as developing antibodies in their blood, patients given the vaccine were found to have developed a robust T cell response - helping their body identify and neutralise the virus.
"The immune system has two ways of finding and attacking pathogens - antibody and T cell responses," said Andrew Pollard, a member of the Oxford team.
"This vaccine is intended to induce both, so it can attack the virus when it's circulating in the body, as well as attacking infected cells."
The Oxford team found that among the 500 or so patients given a single dose of the vaccine - developed jointly with pharmaceutical giant AstraZeneca - their immune response peaked around 14 days and decreased slightly by day 56, the end of the study period.
The other 500 patients were instead given a meningitis vaccine as a placebo.
AstraZeneca shares surged 10 per cent but then gave up most of those gains, to close up 1.45 per cent on the day.
AstraZeneca has signed agreements with governments around the world to supply the vaccine should it prove effective and gain regulatory approval. It has said it will not seek to profit from the vaccine during the pandemic.
AZD1222 was developed by Oxford and licensed to AstraZeneca, which has put it into large-scale, late-stage trials to test its efficacy. It has signed deals to produce and supply over 2 billion doses of the shot, with 300 million doses earmarked for the United States.
Pascal Soriot, Chief Executive of AstraZeneca, said the company was on track to be producing doses by September, but that hopes that it will be available this year hinged on how quickly late-stage trials could be completed, given the dwindling prevalence of the virus in Britain.
Late-stage trials are underway in Brazil and South Africa and are due to start in the United States, where prevalence is higher.
Targeting two doses
The trial results showed a stronger immune response in 10 people given an extra dose of the vaccine after 28 days, echoing a trial in pigs.
Professor Gilbert said the early-stage trial could not determine whether one or two doses would be needed to provide immunity.
"It may be that we don't need two doses, but we want to know what we can achieve," she told reporters.
AstraZeneca's biopharma chief, Mene Pangalos, said the firm was leaning towards a two-dose strategy for later-stage trials, and did not want to risk a single or lower dose that might not work.
The antibody levels generated were "in the region" of those seen in convalescent patients, he said.
Researchers said the vaccine caused minor side effects more frequently than a control group, but some of these could be reduced by taking the painkiller paracetamol.