She said it needed to be shown to work in late stage testing, large quantities needed to be manufactured, and regulators needed to quickly agree to clear it for emergency use before large numbers of people can be vaccinated.
England’s chief medical officer Chris Whitty and his deputy, Jonathan Van-Tam, have expressed differing views on the possible timetable.
“The chances of us getting a vaccine before Christmas that is actually very effective are, in my opinion, very low,” Whitty told lawmakers.
Van-Tam, however, said he was “cautiously optimistic that we will have vaccine this side of Christmas”.
Oxford scientists had envisioned a million doses of the potential vaccine to be produced by September.
Although the deal with AstraZeneca provided the manufacturing capacity to do so, the low prevalence of the novel coronavirus in Britain has complicated the process of proving its effectiveness.
Critical end-stage trials to provide data are underway in Brazil and South Africa and are expected to begin in the United States.
There are no approved vaccines for COVID-19 yet, but the World Health Organization has said AstraZeneca’s vaccine is one of the main candidates.