Oxford begins its human vaccine trial, injects first two volunteers

"Of course, we have to test it and get data from humans. We have to demonstrate it actually works and stops people getting infected with coronavirus before using the vaccine in the wider population."

Prof Gilbert previously said she was "80% confident" the vaccine would work, but now prefers not to put a figure on it, saying simply she is "very optimistic" about its chances.

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.

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.