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.”