The vaccine, called AZD1222 and being developed by AstraZeneca and scientists at Britain's University of Oxford, did not prompt any serious side effects and elicited antibody and T-cell immune responses, according to trial results published in The Lancet medical journal.
"We hope this means the immune system will remember the virus, so that our vaccine will protect people for an extended period," study lead author Andrew Pollard of the University of Oxford said.
"However, we need more research before we can confirm the vaccine effectively protects against SARS-CoV-2 (COVID-19) infection, and for how long any protection lasts," he said.
AstraZeneca's is among the leading vaccine candidates against a pandemic that has claimed more than 600,000 lives, alongside others in mid and late-stage trials.
These include shots being developed by China's Sinovac Biotech, another from state-owned Chinese firm Sinopharm, and one from the U.S. biotech firm Moderna.
AstraZeneca has signed agreements with governments around the world to supply the vaccine should it prove effective and gain regulatory approval. The company has said it will not seek to profit from the vaccine during the pandemic.
Researchers said the vaccine caused minor side effects more frequently than a control group, but some of these could be reduced by taking paracetamol, with no serious adverse events from the vaccine.
(Reporting by Alistair Smout; additional reporting by Pushkala Aripaka and Kate Kelland; Editing by Edmund Blair)