The Oxford coronavirus vaccine shows a strong immune response in adults in their 60s and 70s, raising hopes that it can protect age groups most at risk from the virus.
Researchers say the Lancet phase two findings, based on 560 healthy adult volunteers, are "encouraging".
They are also testing whether the vaccine stops people developing Covid-19 in larger, phase three trials.
Early results from this crucial stage are expected in the coming weeks.
Three vaccines - Pfizer-BioNTech, Sputnik and Moderna - have already reported good preliminary data from phase three trials, with one suggesting 94% of over-65s could be protected from Covid-19.
The UK has already ordered 100 million doses of the Oxford vaccine, manufactured by AstraZeneca, 40 million doses of the Pfizer-BioNTech vaccine and five million of the Moderna vaccine.
Prof Andrew Pollard, study lead from the University of Oxford, told the BBC he was "absolutely delighted with the results" showing a strong immune response "even in those over 70 years of age".
On whether the vaccine protects people against Covid-19, he said they were "not there yet" but the data would probably be released "before Christmas".
Prof Pollard said there "no competition" with other vaccines, adding that multiple vaccines needed to be successful.
"We will need all of them to protect people around the globe," he said.
The challenge with developing a Covid vaccine is to trigger the body to fight back against the virus no matter how old someone is.