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

In Europe, the XBB.1.5 subvariant was 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, the WHO's senior emergency officer for Europe, Catherine Smallwood, said, 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 detected so far - accounted for 27.6 oer cent of COVID-19 cases in the United States for the week ended January 7, health officials have said.

It was unclear if XBB.1.5 would cause its own wave of global infections.

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" and if action is considered, "travel measures should be implemented in a non-discriminatory manner," Smallwood said.

That did not mean the agency recommended testing for passengers from the United States at this stage, she added.

Hans Kluge, the WHO's regional director for Europe, shared the current assessment of the European Union health authority ECDC that the recent surge in coronavirus cases across China will probably not have a major impact on Europe.

Kluge added that this is still no reason for complacency and that countries are free to take necessary precautionary measures to protect their populations.

States that are introducing precautionary travel measures should, however, make sure that they are rooted in science and non-discriminatory, Kluge said.

Measures that could be taken include genomic surveillance and targeting passengers from other countries as long as it does not divert resources from domestic surveillance systems.

Others include monitoring wastewater around points of entry such as airports.

XBB.1.5 is 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, itself a recombinant of two other Omicron subvariants.

Concerns about XBB.1.5 fuelling a fresh spate of cases in the US and beyond are rising amid a surge of COVID-19 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.

The WHO is aware that the case-definition of what counts as a COVID-19 death in China is narrow and "not necessarily the case definition that WHO has recommended countries adopt," Smallwood said.

More than a dozen countries - including the US - are demanding COVID-19 tests from travellers from China.

with DPA