A coronavirus variant first identified in Britain is rapidly spreading in the US, threatening to bring a surge of new cases as its prevalence doubles about every 10 days, a new study shows.
The paper was posted online on Sunday and has not yet been reviewed by peers, but it offers the most comprehensive look at the rise of the B117 strain in the country hit hardest by the pandemic.
A team of scientists led by researchers at the Scripps Research Institute analysed half a million test samples collected across the country since last summer.
Rather than individually sequencing them, they were able to identify a particular anomaly that was a "reliable proxy" for B117.
They also analysed the full genetic sequence, a more time-consuming process, for 212 samples.
They found the variant was introduced at several points into the US in November 2020, and while low in overall frequency, it is set to become the dominant form of the virus by March.
The team said the transmission rate was up to 45 per cent higher than more common variants and its prevalence is doubling every week and a half.
Britain saw devastating waves of Covid after B117 became dominant, and the strain has been observed in many European countries including Portugal and Ireland.
"B117 is much more contagious so it can quickly overwhelm a nation," Ashish Jha, dean of the Brown University School of Public Health, tweeted in response to the paper.
Ireland had its outbreak under control by late 2020, but in January, B117 started an exponential wave from which the country is now recovering.
The US has had the world's largest outbreak, with more than 27 million confirmed cases and 460,000 deaths.
But its last wave peaked around January 8 and infections have been dropping since.
There are fears that B117 could start a new surge and it is spreading particularly rapidly in Florida, the study said.
The study's authors called for the US to build up its Covid-19 genomic surveillance system.
"I'm asking everyone to please keep your guard up," Rochelle Walensky, director of the Centres for Disease Control and Prevention, said on Monday.
"The continued proliferation of variants remains of great concern and is a threat that could reverse the recent positive trends we are seeing."
Current vaccines are effective against the variant, while the use of masks drastically reduces transmission.
"The two things that we can do are make sure we adhere to the public health measures, and get as many people vaccinated as quickly as we possibly can," said Dr Anthony Fauci, President Joe Biden's chief medical adviser.
