A new Covid sub-variant is causing some concern in the US, where it is spreading rapidly.
Some cases have been recorded in the UK, so what do you need to know about XBB.1.5?
What is XBB.1.5?
It is yet another offshoot of the globally dominant Omicron Covid variant, which itself followed the earlier alpha, beta, gamma and delta variants.
Omicron has outperformed all previous versions of coronavirus since it emerged in late 2021, and has given rise to many sub-variants which are even more contagious than the original.
Symptoms of XBB.1.5 are thought to be similar to those of previous Omicron strains, but it's still too early to confirm this. Most people experience cold-like symptoms
Is XBB.1.5 more infectious or dangerous than earlier variants?
XBB.1.5 itself evolved from XBB, which began circulating in the UK in September 2022, but which has not been classified as a so-called "variant of concern" by health authorities.
XBB had a mutation that helped it beat the body's immune defences, but this same quality also reduced its ability to infect human cells.
Prof Wendy Barclay from Imperial College London said XBB.1.5 had a mutation known as F486P, which restores this ability to bind to cells while continuing to evade immunity. That makes it spread more easily.
She said these evolutionary changes were like "stepping stones", as the virus evolves to find new ways of bypassing the body's self-defence mechanisms.
Scientists from the World Health Organization (WHO) confirmed on Wednesday that XBB.1.5 has a "growth advantage" above all other sub-variants seen so far.
But they said there was no indication it was more serious or harmful than previous Omicron variants.
The WHO said it would keep a close watch on lab studies, hospital data and infection rates to find out more about its impact on patients.
Where is XBB.1.5 spreading?
Over 40% of Covid cases in the United States are estimated to be caused by XBB.1.5, making it the dominant strain in the country.
At the beginning of December, it accounted for only 4% of cases so it has quickly overtaken other versions of Omicron.
Covid hospital admissions have been rising in recent weeks across the US.
The UK Health Security Agency is due to release a report on variants spreading in the UK next week, and may refer to XBB.1.5.
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