The vaccine is made from a weakened version of a common cold virus (known as an adenovirus) from chimpanzees that has been modified so it cannot grow in humans.
The Oxford team has already developed a vaccine against Mers, another type of coronavirus, using the same approach – and that had promising results in clinical trials.
How will they know if it works?
The only way the team will know if the Covid-19 vaccine works is by comparing the number of people who get infected with coronavirus in the months ahead from the two arms of the trial.
That could be a problem if cases fall rapidly in the UK, because there may not be enough data.
Prof Andrew Pollard, director of the Oxford Vaccine Group, who is leading the trial, said: “We’re chasing the end of this current epidemic wave. If we don’t catch that, we won’t be able to tell whether the vaccine works in the next few months. But we do expect that there will be more cases in the future because this virus hasn’t gone away.”
The vaccine researchers are prioritising the recruitment of local healthcare workers into the trial as they are more likely than others to be exposed to the virus.
A larger trial, of about 5,000 volunteers, will start in the coming months and will have no age limit.
Older people tend to have weaker immune responses to vaccines. Researchers are evaluating whether they might need two doses of the jab.
The Oxford team is also considering a vaccine trial in Africa, possibly in Kenya, where the rates of transmission are growing from a lower base.
If the numbers could be a problem, why not deliberately infect volunteers with coronavirus?
That would be a quick and certain way to find out if the vaccine was effective, but it would be ethically questionable because there are no proven treatments for Covid-19.
But that might be possible in the future. Prof Pollard said: “If we reach the point where we had some treatments for the disease and we could guarantee the safety of volunteers, that would be a very good way of testing a vaccine.”