A new COVID variant could be the one to "watch out for" this year after a surge in cases in the US, an expert has said.
Tim Spector, professor of genetic epidemiology at King's College London, highlighted an Omicron variant after a scientist said cases had more than doubled in America in a week.
More than 40% of COVID cases in the US are now caused by the XBB.1.5 variant, according to data from the Centres for Disease Control and Prevention (CDC) from Friday.
In the northeast of the US, about 75% of confirmed cases are reported to be XBB.1.5.
US scientist Eric Topol said it is now out-competing all variants and "we've not seen such rapid growth of a variant" since the original Omicron emerged a year ago.
In response, Prof Spector, the founder of the COVID Zoe app, tweeted: "XBB could be the new variant to watch out for .. in 2023."
XBB.1.5 is a mutated version of Omicron XBB, which was first detected in India in August.
XBB has been found in at least 70 countries, according to the World Health Organisation, causing surges of infection in some parts of Asia, including India and Singapore, in October.
Studies have found that the strain is capable of evading antibodies from previous COVID infections or vaccinations.
Concern about XBB.1.5 is largely based on how it is currently spreading in the US and it has not been listed as a variant of concern by the UK Health Security Agency (UKHSA).
Some 4% of all UK COVID cases were of the XBB 1.5 variant, figures from Cambridge's Sanger Institute show for the week to Saturday 17 December.
'A wake-up call'
Dr Meera Chand, director of clinical and emerging infection at the UKHSA, said: "It is not unexpected to see new variants of SARS-CoV-2 emerge. UKHSA is monitoring the situation closely, as always.
"Vaccination remains our best defence against future COVID-19 waves, so it is still as important as ever that people come take up all the doses for which they are eligible as soon as possible."
But Professor Lawrence Young, a virologist at Warwick University, said the new strain is a "wake-up call" and could exacerbate the NHS crisis.
He told Mail Online: "We don't know how this variant is going to behave in the UK in a population that has been previously exposed to other Omicron variants and where many of the over 50s have had booster shots with a bivalent vaccine.
"Nevertheless, this is a wake-up call - a sharp reminder that we can't be complacent about COVID.
"The threat of XBB.1.5 and other COVID variants further exacerbating the current NHS crisis stresses the need for us to remain vigilant."
Dr Barbara Mahon, from the CDC, said there is no indication the XBB.1.5 variant causes more severe illness than any other Omicron variants.
She told NBC News: "We're seeing hospitalisations have been notching up overall across the country. They don't appear to be notching up more in the areas that have more XBB.1.5."
New variant 'the next big one'
Although many public health experts are expressing concern about the rising COVID cases in China - with the UK among the countries introducing new testing restrictions - infectious disease experts have been increasingly worried about the XBB.1.5 variant.
Dr Michael Osterholm, an infectious disease expert at the University of Minnesota, said: "Ironically, probably the worst variant that the world is facing right now is actually XBB."
US epidemiologist Eric Feigl-Ding also called XBB.1.5 "the next big one", saying it is "both more immune evasive & better at infecting" than previous variants.
Dr Isaach Bogoch, an infectious disease physician and epidemiologist at the University of Toronto, said XBB.1.5 clearly has "immune evasive properties" and "we might certainly have a wave".
But he said if this happens, it is less likely to be as deadly or overwhelming to health care systems as previous waves because immunity has been built up.