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 (WHO) officials said on Tuesday.

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

Passengers should be advised to wear masks in high-risk settings such as long-haul flights, said the 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.”

XBB.1.5 — the most transmissible Omicron subvariant that has been detected so far — accounted for 27.6 per cent of Covid-19 cases in the United States for the week ending January 7, US 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”, Smallwood added, saying it was crucial not to focus exclusively on one particular geographic area.

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

That did not mean the agency recommended testing for 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 does not divert resources away from domestic surveillance systems. Others include wastewater monitoring systems around points of entry, such as airports.

XBB.1.5 is yet another descendant of Omicron, the most contagious — and now globally dominant — variant of the virus that causes Covid-19. It is an offshoot of XBB, first detected in October, which is itself a recombinant of two other Omicron subvariants.

Concerns about XBB.1.5 fuelling a fresh spate of cases in the United States and beyond are on the rise at the same time as a surge of Covid cases in China, after the country pivoted away from its signature “zero Covid” policy last month.

According to data reported by the WHO earlier this month, an analysis by the Chinese Centre for Disease Control and Prevention showed a predominance of Omicron sublineages BA.5.2 and BF.7 among locally acquired infections.

Many scientists — including from the WHO — believe China is likely under-reporting the true extent of its outbreak.

More than a dozen countries — including the United States — are demanding Covid tests from travellers from China.