The Prime Minister has refused to rule out a delay in easing lockdown restrictions if the South African variant continues to spread.
Boris Johnson said that if the variant reduces the Oxford vaccine's effect to stop the spread of coronavirus that more restrictions could be needed.
Earlier today Dr Mike Tildesley, who advises the Government as a member of the Scientific Pandemic Influenza Group on Modelling (Spi-M), said it was "very possible" the South African variant is already quite widely spread in the UK.
Dr Tildesley said lockdown restrictions may be needed for longer if the variant does turn out to be prevalent in the UK.
He said: "If that is the case and actually people can still get infected and still pass on the infection with the South African variant, and of course if it does become widespread across the country, then that has significant implications because it means that even with high levels of vaccination there will be a lot of people that could potentially get infected and could potentially pass it on and it may mean that more restrictions might be needed for longer if we can't get on top of this."
Boris Johnson said that the government is confident the vaccines will stop serious disease and death.
Pressed on whether there could be a delay to easing restrictions if the jab is less effective at reducing transmission of the South African variant, the Prime Minister said: "We think that all the vaccines that we're using, both the vaccines that we're currently using, are effective in stopping serious disease and death.
"We also think, particularly in the case of the Oxford/AstraZeneca vaccine, that there is good evidence that it is stopping transmission as well, I think 67% reduction in transmission with the Oxford/AstraZeneca vaccine.
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"They remain of massive benefit to our country and to the population as we go through the pandemic and I've no doubt that vaccines generally are going to offer a way out.
"With every day that goes by you can see that medicine is slowly getting the upper hand over the disease."