Concerns continue to mount over the spread of the B.1.617.2 variant, which was first identified in India.
Speaking at a Downing Street press conference on Friday, Johnson insisted "present evidence" does not suggest a need to delay the road map for unlocking. This will see restrictions further eased on Monday by allowing indoor mixing.
However, he added: “But I have to level with you that this new variant could pose a serious disruption to our progress and could make it more difficult to move to step four in June.”
The prime minister added that if the variant proves to be “significantly more transmissible” than other strains, “we’re likely to face some hard choices”.
Johnson also announced an acceleration of the vaccine programme due to concerns over the variant. It means second doses for the over-50s and clinically vulnerable will be given eight weeks after the first dose, rather than 12.
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On Thursday, Johnson had admitted he is "anxious" about the variant and that he's "ruling nothing out", including local lockdowns.
The latest data show UK cases of the Indian variant have risen by 152% in the space of a week.
Figures from Public Health England (PHE) showed that as of Wednesday, there were 1,313 confirmed cases involving the B.1.617.2 variant. This was up 793 from 520 last week.
The variant is said to be as transmissible as the Kent variant which caused the massive spike of infections, hospital admissions and deaths in December and January – though at that point only a small number of people had been vaccinated.
There is currently no solid data about the impact the variant may have on vaccines, but England’s chief medical officer Prof Chris Whitty said on Monday: “At this point in time our view is it is less likely to be able to escape vaccination than some of the other variants, particularly the South African one."
Shortly before the Downing Street press conference on Friday, Nicola Sturgeon announced that following COVID outbreaks, Glasgow and Moray will remain under Level 3 restrictions in Scotland as the rest of the country sees rules eased to Level 2.
The first minister said initial research suggested the outbreak in Glasgow is possibly being driven by the Indian variant.
Initially, both areas will remain at Level 3 for a week, with a further decision made at the end of next week.
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