XBB.1.5 was first identified in the US in October and catapulted to people's attention in the new year when it was revealed 40% of infections in the country were caused by the variant.
What's behind the name XBB.1.5 and the nickname 'kraken'?
The nickname kraken has quickly gained traction online, perhaps because of how well the image of a mythological monster matches a fiendishly evolving virus.
It appears to have first been suggested for the COVID variant on Twitter by evolutionary biologist Professor T. Ryan Gregory.
"This year, some of us decided that we needed nicknames for variants worth watching, given that the WHO wasn't giving any new names under their system," he wrote.
"We've been using mythological creature names for variants that are being discussed outside of technical discussions."
Originating in Scandinavian folklore, the kraken is a many-tentacled sea monster that earned its place in legend for destroying entire ships and dragging sailors to their death.
XBB.1.5 is a sub-variant of Omicron, which is why it does not have a Greek letter of its own.
Professor Gregory explained you can think of Greek letters as groups of species, with Omicron like "mammals" and Delta like "birds". He added: "That doesn't mean everything is 'still just Omicron'. Whales and bats are both mammals but are very different biologically."
XBB.1.5 gets its name through the Pango method of naming (Pango stands for Phylogenetic Assignment of Named Global Outbreak) and the X indicates it's a recombinant variant.
This is a hybrid of two variants that is formed when both infect one person at the same time, and recombine to make something new.
Is XBB.1.5 more serious or worrying than other variants?
The World Health Organisation (WHO) has said XBB.1.5 is "the most transmissible sub-variant which has been detected yet".
XBB.1.5 is a mutated version of Omicron XBB, which was first detected in India in August. XBB.1.5 has now been found in 25 countries.
XBB can evade the body's immune system and XBB.1.5 is just as skilled at doing so - with the added bonus (from the virus's perspective) of being better at "binding" to cells, meaning it can spread more easily.
Dr Maria Van Kerkhove, the WHO's technical lead for COVID, told a press conference on Wednesday: "We are concerned about its growth advantage, in particular in some countries in Europe and in the US.
"Our concern is how transmissible it is… and the more this virus circulates, the more opportunities it will have to change."
She said there is no data yet to prove that XBB.1.5 causes more severe disease, but added the WHO is working on a new risk assessment of the variant that it expects to release soon.
What about long COVID?
With the variant more easily spread, it is likely more people will get infected, which could lead to severe outcomes, including long COVID.
What have we seen in the UK so far?
One in 25 COVID cases - 4% - were of the XBB.1.5 variant, figures from Cambridge's Sanger Institute show for the week to Saturday 17 December.
The US saw the so-called kraken variant go from making up 4% to 40% of COVID cases in the space of a few weeks, and fears of something similar happening here have prompted warnings it could be the "new variant to watch out for".