Highly contagious new Covid variant XBB.1.5 is behind 40 per cent of cases in the US as it sweeps across both sides of the Atlantic
In the UK, the Omicron variant is now responsible for one in 25 cases.
Latest data from the US Centres for Disease Control and Prevention showed cases doubling from the previous week.
It comes as spiralling cases in China have led a number of countries, including the UK and US, to request negative pre-departure Covid tests for travellers from the country.
Dr Michael Osterholm, an infectious disease expert at the University of Minnesota, is warning the XBB.1.5 variant is becoming a major threat.
“Ironically, probably the worst variant that the world is facing right now is actually XBB,” he said.
Dr Osterholm said that seven of the 10 US states where cases and hospitalisations are rising are in the north-east, concurrent with an increase of XBB cases there, he said.
Combinations of the XBB and XBB.1.5 variants, together accounted for 44.1 per cent of the total cases in the US for the week ending December 31.
For the week ending December 24, XBB.1.5 had made up 21.7 per cent of the total cases.
“Every previous two winters, we have seen a surge of infections that peaked in mid January and I expect the same to happen this year,” said John Moore, a professor of microbiology and immunology at Weill Cornell Medical College in New York.
He added that the difference between the last two years and this year is that there has not been a significant increase in the death rates.
The XBB variant has been driving up cases in parts of Asia, including Singapore.
The strain was first detected in India in August.
Experts say it is “worrying” and have raised concerns that vaccines may not be effective.
“The situation in China makes us very worried,” said Wilbur Lam, who runs the US National Institutes of Health’s RADx Tech Testing Validation Core.
Last month researchers from Columbia University published a study raising concerns that the new subvariants of Omicron “present serious threats to current Covid-19 vaccines, render inactive all authorised antibodies, and may have gained dominance in the population because of their advantage in evading antibodies”.
Last week US regulators said the mutations have rendered at least one Covid test unreliable.
In general, scientists are finding that it takes a bit longer for tests to turn positive when omicron infections are present, Mr Lam said.