The coronavirus variant first identified in Britain is now the most common Covid-19 strain in the US, the head of the Centres for Disease Control and Prevention said on Wednesday.
"Based on our most recent estimates from CDC surveillance, the B117 variant is now the most common lineage circulating in the United States," agency chief Dr Rochelle Walensky said.
The US has had weeks of slow but steady growth in new coronavirus infections, due in part to the spread of such "variants of concern".
She said daycare centres and youth sports helped to accelerate the rise in infections, and warned of an increase in younger adults suffering serious symptoms.
With the CDC recording a seven-day average of more than 63,000 daily cases, nearly comparable to the summer surge the US experienced in 2020, officials say the country is in a race between vaccines and variants.
The US is averaging 3 million Covid-19 vaccinations a day.
"Testing remains an important strategy to rapidly identify and isolate infectious individuals, including those with variants of concern," Dr Walensky said.
The US lags behind Britain and other nations when it comes to sequencing virus strains, especially during the pandemic.
Now under President Joe Biden's administration, the CDC says it is boosting its capabilities.
"We’re doing more surveillance and sequencing so we can understand what is happening with B117 and variants," Dr Walensky said.
Florida, Michigan and Minnesota top the US in the number of UK variant cases.
Considerable focus has been placed on Michigan which, with its rapid rise in cases, could be a precursor for outbreaks in other states.
The CDC said it had a team investigating the state's outbreak.