(Bloomberg) -- Chris Whitty, U.K. Prime Minister Boris Johnson’s senior medical adviser, said there is only a slim chance of an effective coronavirus vaccine being available by Christmas, even as the University of Oxford reported progress on its initiative.

“No one should be under any illusions: the chance of us getting a vaccine before Christmas that actually is highly effective are, in my view, very low,” Whitty told a committee of lawmakers on Tuesday. “Low probability for this winter flu season.”

Whitty’s downbeat assessment came despite the Oxford vaccine effort showing promising results in early human testing, and is a sign of ongoing uncertainty in the scientific community over the likelihood of quick success. Whitty has previously warned of the risk of a second spike of the virus in the winter, coinciding with a flu outbreak.

This week, the U.K. signed agreements to buy 90 million doses of vaccines in development by drugmakers including Pfizer Inc. and Valneva SE, in addition to a supply agreement it already has with AstraZeneca Plc for the Oxford vaccine.

Whitty’s deputy, Jonathan Van-Tam, told the same committee that he is more optimistic about a vaccine being ready by Christmas, but in any event it would not be possible to administer it close to the flu vaccine.

“We won’t have any data on whether it can be given at the same time,” Van-Tam said. “Likely we will have to delay or separate those two vaccines apart in terms of the staging we give them to the relevant patients.”

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