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.