The experimental vaccine, which has been licensed to AstraZeneca produced an immune response in early-stage clinical trials, data showed on Monday, preserving hopes it could be in use by the end of the year.
"The end of the year target for getting vaccine rollout, it's a possibility but there's absolutely no certainty about that because we need three things to happen," said Sarah Gilbert, adding it needed to be shown to work in late stage trials, there needed to be large quantities manufactured and regulators had to agree quickly to licence it for emergency use.
"All of these three things have to happen and come together before we can start seeing large numbers of people vaccinated."