An ongoing trial involving 1077 healthy adults found that the vaccine "induced strong antibody and T cell immune responses" up to day 56, the researchers wrote in The Lancet medical journal.

They said the immune responses "may be even greater after a second dose," according to a trial with a sub-group of 10 participants.

The British government has already ordered 100 million doses of the potential vaccine, which is among dozens of vaccine candidates worldwide.

"The early stage trial finds that the vaccine is safe, causes few side effects, and induces strong immune responses in both parts of the immune system," the Oxford researchers said.

They said the vaccine produces a cellular immune response, or T cell response, within 14 days of vaccination and an antibody response within 28 days.

The vaccine uses a genetically modified common cold virus that infects chimpanzees, weakened so that it can't cause disease in humans, said Oxford's Andrew Pollard, the lead author of the study.

"The immune system has two ways of finding and attacking pathogens - antibody and T cell responses," Pollard said.

"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."

"We hope this means the immune system will remember the virus, so that our vaccine will protect people for an extended period," he said.

Pollard cautioned that his team needs "more research before we can confirm the vaccine effectively protects against Sars-CoV-2 infection, and for how long any protection lasts".

Sars-CoV-2 is the scientific name for the coronavirus that causes the respiratory illness COVID-19.

Australian Associated Press