Countries should consider recommending passengers wear masks on long haul flights to counter the latest Omicron subvariant of COVID-19 given its rapid spread in the United States, World Health Organisation officials said on Tuesday.

In Europe, the XBB.1.5 subvariant is being detected in small but growing numbers, WHO/Europe officials said in a press briefing.

Passengers should be recommended to wear masks in high-risk settings such as long-haul flights, said WHO’s senior emergency officer for Europe, Catherine Smallwood, adding “this should be a recommendation issued to passengers arriving from anywhere where there is widespread COVID-19 transmission”.

The XBB.1.5 subvariant – the most transmissible Omicron sub-variant that has been detected so far – accounted for 27.6% of COVID-19 cases in the United States for the week ending Jan. 7, U.S. health officials have said.

It remains unclear if XBB.1.5 will cause its own wave of infections around the world. Current vaccines continue to protect against severe symptoms, hospitalisation and death, experts say.

“Countries need to look at the evidence base for pre-departure testing”, Smallwoood noted, adding it was crucial to not be “blindsided” by an exclusive focus on one particular geographic area.

If travel measures are considered, she said, “our opinion is that travel measures should be implemented in a non-discriminatory manner.”

That did not mean the agency recommends the testing of passengers coming from the United States at this stage, she added.

Measures that can be taken include genomic surveillance, and targeting passengers arriving from other countries as long as it not divert resources away from the domestic surveillance systems. Other examples include wastewater monitoring systems that can look at wastewater around points of entry such as airports.

XBB.1.5 is yet another descendant of Omicron, the most contagious variant of the virus causing COVID-19 that is now globally dominant. It is an offshoot of XBB, first detected in October, which is itself a recombinant of two other Omicron sub-variants.