The highly contagious UK COVID-19 variant could become the dominant strain in the US by March, health officials said.
The Centers for Disease Control and Prevention (CDC) warned of “rapid growth” of the variant in coming weeks.
It said such a spike could further threaten health systems already strained by a winter Covid surge.
The warning came on Friday as President-elect Joe Biden unveiled an ambitious plan to ramp up vaccinations.
To meet his target of inoculating 100 million Americans within his first 100 days in office, Mr Biden said his administration would take a more active role in stepping up the distribution of vaccines.
He outlined a plan to set up new mass vaccination centres, hire extra health workers, and ensure the shot is available to everyone, including minority communities that have been hit hardest by the epidemic.
Official data shows that, so far, 12.2 million vaccine doses of have been administered in the US – a figure Mr Biden has criticised as insufficient. More than 30 million doses have been distributed to states.
In a speech on Friday, Mr Biden told Americans that “we remain in a very dark winter”, admitting that “things will get worse before they get better”.
“This is going to be one of the most challenging operational efforts ever undertaken by our country,” Mr Biden, who takes office on 20 January, said of the vaccination drive.
His address came a day after he announced a $1.9tn (£1.4tn) stimulus package for the battered US economy that included a further $20bn for the vaccine roll-out.
The plan will need to pass Congress.
The US has recorded the highest number of confirmed coronavirus infections – 23.5 million – of any country in the world.
At about 391,000, the country’s coronavirus deaths account for a fifth of the global total, which passed the two-million mark on Friday.
The crisis is particularly acute in the state of California, where deaths have surged by more than 1,000% since November.
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