GENEVA (NYTIMES) - The World Health Organisation (WHO) reported that the highly contagious omicron subvariant, BA.2, that is helping to drive another surge of coronavirus cases in Europe is now the dominant version of Omicron around the world.

Globally, BA.2 made up about 86 per cent of cases reported to the WHO from Feb 16 to March 17, the agency said in a report on Tuesday (March 22).

The previously dominant subvariants, BA.1 and BA1.1, together represented about 13 per cent of the cases.

BA.2 is already dominant in the WHO's Americas region, and its share of cases has been steadily increasing in parts of Africa, Asia, Europe and the Middle East since the end of 2021, the agency said.

When the WHO last reported these figures March 8, it said that BA.1.1 was the dominant subvariant and that BA2 made up 34 per cent of new cases.

In the United States, about a third of new coronavirus cases are BA.2, Dr Rochelle Walensky, the director of the Centres for Disease Control and Prevention, said at a White House briefing on Wednesday.

US health officials have said that they expect case numbers to rise, but that they do not anticipate a major surge caused by BA.2.

While BA.2 is more transmissible than BA.1, it has not been shown to cause more severe illness.

And even though the virus has evolved considerably since vaccines against it were first developed, the inoculations still work, Dr Maria Van Kerkhove, the WHO's Covid-19 technical lead, said in an interview posted on the agency's website on Tuesday.

"Our vaccines remain incredibly effective at preventing severe disease and death, including against both of the sublineages of BA.1 and BA.2," she said.

Scientists suspect that BA.2's rapid growth is due to its unique mutations. In the gene for the spike protein on the surface of the virus, BA.2 has eight mutations not found in BA1.

Although BA2 has become the latest subvariant on many people's minds, there are also three so-called recombinant variants that the WHO has deemed noteworthy enough to be named. One of these variants, nicknamed "deltacron", was discovered in February, but had not been officially named.

On Tuesday, the agency said that it had named the three variants - two versions of deltacron and one that combined BA.1 and BA.2 - XD, XE and XF.

There was no evidence that these recombinant variants are more transmissible or cause "more severe outcomes", the report said.

Van Kerkhove said that, over the past two years, virus surveillance, testing and sequencing have helped countries implement public health measures that have evolved with the virus.

Her statement came on the same day that a senior WHO official in Europe said that cases have surged in the region because authorities were too quick to relax pandemic restrictions.

Rather than take a gradual, measured approach, the countries "are lifting those restrictions brutally, from too much to too few", said the official, Dr Hans Kluge, the organisation's regional director for Europe.

Kluge added that the increase in new cases was linked to the spread of BA.2.