Countries should consider advising travellers to wear face masks on long-haul flights to curb the spread of the most infectious COVID-19 variant yet, the World Health Organization (WHO) has said.
The latest Omicron offshoot, XBB.1.5, has been spreading rapidly across the US, accounting for 27.6% of the nation's cases last week.
The variant was behind nearly 8% of cases in the UK in the two weeks to 2 January, data from GISAID and CoVariants.org suggests, and its improved ability to evade immunity means that number could quickly rise.
Although XBB.1.5 is more effective at infecting cells than its predecessors, it remains unclear if it will cause its own wave of cases around the world.
There is no evidence to suggest yet that it is more dangerous, and vaccines still offer good protection against severe illness and death, experts say.
In light of the subvariant being detected in small but growing numbers in Europe, the WHO suggested passengers on long-haul flights could wear face masks as a safety measure.
The WHO's senior emergency officer for Europe, Catherine Smallwood, added: This should be a recommendation issued to passengers arriving from anywhere where there is widespread COVID-19 transmission."
Countries must "look at the evidence base for pre-departure testing", she said, adding that it is crucial not to be "blindsided" by an exclusive focus on one particular geographic area.
It comes as a number of nations, including the UK, to require testing for people arriving from mainland China, in response to a huge surge in cases after the abrupt lifting of its zero-COVID policy.
If travel measures are considered, Smallwood said: "Our opinion is that travel measures should be implemented in a non-discriminatory manner."
She said this did not mean the agency recommends testing passengers coming from the US at this stage.
While countries to not have to follow the WHO's recommendations, they are a sign of growing concern about future waves of COVID-19.
Infections in England hit a six-month high in late December, with one in 20 people – nearly three million – likely to have the virus over the Christmas period, according to the Office for National Statistics.
This has added to the immense pressure faced by the NHS, which is also tackling its worst flu season for a decade, and a wave of Strep A which has killed 29 children in the UK so far.
The health service is facing record waits in A&E, while a shortage of hospital beds has meant patients have been forced to wait for hours in corridors and in ambulances parked outside.