A sub-variant of the highly contagious Omicron coronavirus strain, which some studies indicate could be even more infectious than the original version, has been detected in 57 countries, the WHO said Tuesday.

The fast-spreading and heavily mutated Omicron variant has rapidly become the dominant variant worldwide since it was first detected in southern Africa 10 weeks ago.

In its weekly epidemiological update, the World Health Organization said that the variant, which accounts for over 93 percent of all coronavirus specimens collected in the past month, counts several sub-lineages: BA.1, BA.1.1, BA.2 and BA.3.

The BA.1 and BA.1.1 -- the first versions identified -- still account for over 96 percent of all the Omicron sequences uploaded to the GISAID global science initiative, it said.

But there has been a clear rise in cases involving BA.2, which counts several different mutations from the original -- including on the spike protein that dots the virus's surface and is key to entering human cells.

"BA.2- designated sequences have been submitted to GISAID from 57 countries to date," WHO said, adding that in some countries, the sub-variant now accounted for over half of all Omicron sequences gathered.

The UN health agency said little was known yet about the differences between the sub-variants, and called for studies into its characteristics, including its transmissibility, how good it is at dodging immune protections and its virulence.

Several recent studies have hinted that BA.2 is more infectious than the original Omicron.

Maria Van Kerkhove, one of the WHO's top experts on Covid, told reporters Tuesday that information about the sub-variant was very limited, but that some inital data indicated BA.2 had "a slight increase in growth rate over BA.1" 

Omicron in general is known to cause less severe disease than previous coronavirus variants that have wreaked havoc, like Delta, and Van Kerkhove said there so far was "no indication that there is a change in severity" in the BA.2 sub-variant.

She stressed though that regardless of the strain, Covid remained a dangerous disease and people should strive to avoid catching it.

"We need people to be aware that this virus is continuing to circulate and its continuing to evolve," she said.

"It's really important that we take measures to reduce our exposure to this virus, whichever variant is circulating."

Pfizer vaccine for children under 5

Pfizer and BioNTech said Tuesday they are seeking emergency authorization from US health regulators for use of their Covid vaccine for children aged over six months and under five years.

If the Food and Drug Administration (FDA) authorizes the two-shot regimen, it will become the first Covid vaccine available to this age group in the United States.

The companies said Tuesday that they started submitting their formal application "following a request" from the FDA, which seemingly wants to get the process moving quickly.

In a tweet soon after the announcement, the FDA said it will hold a meeting in two weeks, on February 15, to consider the request.

"Having a safe and effective vaccine available for children in this age group is a priority for the agency," said interim FDA head Janet Woodcock, who added that the FDA asked for the application "in light of the recent Omicron surge."

The companies are seeking authorization for only two doses of their vaccine, but believe a third will be needed "to achieve high levels of protection against current and potential future variants," Pfizer CEO Albert Bourla said in a statement.

"If two doses are authorized, parents will have the opportunity to begin a Covid-19 vaccination series for their children while awaiting potential authorization of a third dose," he added.

To limit side effects for this young age group, Pfizer chose to significantly decrease the dosage of its vaccine, opting for only three micrograms per jab versus 30 for those over 12 years old, and 10 for ages five to 11.

The company's researchers concluded last fall that low doses of the vaccine provided protection in children up to two years old but not in those aged two to five, and announced in December they would add a third dose to their trials.

Data on the three-dose regimen is "expected in the coming months and will be submitted to the FDA to support a potential expansion" of this initial request, Pfizer and BioNTech said in the statement.

If the FDA authorizes the emergency use of the vaccine, an advisory committee for the Centers for Disease Control and Prevention (CDC) will then meet to decide to recommend its use or not.

- Slower uptake among minors -

There are about 23 million children under the age of five in the United States, and two years after the pandemic began, many parents are waiting impatiently to vaccinate their young children against the coronavirus.

The Pfizer-BioNTech vaccine was approved three months ago for emergency use in children from five to 11, but uptake has been relatively slow.

In that age group, only 30 percent have received at least one dose and around 22 percent are fully vaccinated, according to the Centers for Disease Control and Prevention (CDC).

Young children are less susceptible to falling seriously ill with Covid-19, but some nonetheless end up hospitalized by the virus.

New pediatric Covid hospitalizations hit a record high in the United States in December as the Omicron strain spread rapidly.

Some cases of the life-threatening illness MIS-C (multi-system inflammatory syndrome in children), have also been reported in young children with Covid.

CDC data shows that about 400 children aged four and under have died from Covid-19 in the United States since the pandemic began. 

Denmark unmasks to 'live normally' again

Denmark waved goodbye to facemasks and health passes Tuesday as it became the first European Union country to lift all of its domestic Covid curbs despite record numbers of cases of the milder Omicron variant.

Only a few restrictions remain in place at the country's borders, for unvaccinated travellers arriving from non-Schengen countries.

After a first attempt at lifting restrictions between September and November, the Scandinavian country once again ditched its Covid checks and limited opening hours for bars and restaurants.

"For me, the best part is that we don't need to wear masks anymore," Natalia Chechetkina, a receptionist in Copenhagen, told AFP.

"At least now we have a choice -- if we want to protect ourselves or we want to feel free."

Marie Touflet, a 23-year-old French student in the capital, said it was "pretty strange to take the metro without a mask, compared to France".

"It's really nice to be able to see people's faces and it feels like we're living normally again," she said.

The easing comes as Denmark registers around 40,000-50,000 new Covid cases a day, or almost one percent of the country's 5.8 million inhabitants.

Health officials believe those figures will soon start going down.

"There are strong indications that the infection has peaked in the areas where it has been most pronounced," Tyra Krause of Denmark's public health and research institution SSI told news agency Ritzau.

"So it's super good timing for the restrictions to be eased."

- 'Shift of responsibility' -

More than 60 percent of Danes have received a third dose of a Covid-19 vaccine -- one month ahead of health authorities' schedule -- compared to an EU average of just under 45 percent.

Including those who have recently had Covid, health authorities estimate that 80 percent of the population is protected against severe forms of the disease.

"With Omicron not being a severe disease for the vaccinated, we believe it is reasonable to lift restrictions," epidemiologist Lone Simonsen of the University of Roskilde told AFP.

Two years after the start of the pandemic, the Danish strategy enjoys broad support at home.

In a poll published Monday by daily Politiken, 64 percent of Danes surveyed said they had faith in the government's Covid policy, while the lifting of restrictions also has widespread support in parliament.

Going forward, Danes are being urged to exercise personal responsibility.

"Without a Covid pass there will be a shift of responsibility," Simonsen said.

Danes have increasingly used home tests to detect infection, but these are now being phased out and instead, anyone with symptoms is advised to stay home.

The Danish Health Authority currently "recommends" those who test positive to isolate for four days, while contact cases no longer need to quarantine.

Facemasks and the Covid pass are also recommended for hospital visits.

Denmark lifted restrictions as the World Health Organization chief Tuesday warned it was too early for countries to either declare victory over Covid-19 or give up attempts to halt transmission.

- 'Bit of an experiment' -

One of the rare critics in Denmark was the country's Gout Association.

"We think it's important to continue using masks as long as the infection is spreading widely," association director Mette Bryde Lind told Ritzau.

Eskild Petersen, an infectious disease specialist, said the Danish strategy was "a bit of an experiment".

"I would have liked for us to wait 10-14 days after changing quarantine rules before abandoning masks and opening up nightlife" again, he told public television broadcaster TV2.

The government said it does not expect to have to revert to new closures but has remained cautious.

"We can't provide any guarantees when it comes to biology," Prime Minister Mette Frederiksen said last week, announcing the country's return "to life as we knew it before corona".

This is the second time Denmark has tried to return to a pre-pandemic lifestyle.

On September 10, the country lifted all its restrictions before reintroducing some in November.

Museums, cinemas and theatre and concert venues then closed over Christmas and the New Year.

According to the World Health Organization, 73 percent of the European population has contracted Covid-19 at least once since January 2020.

Faced with a lower level of hospitalisations than in previous waves, several European countries have announced a reduction of their restrictions, despite record or very high cases.

"Populations in most countries have reached high levels of immunity, from vaccines or natural illness", Simonsen said.

"This is how it ends."

Norway scraps most Covid curbs

Norway on Tuesday announced it would scrap most of its Covid restrictions despite an Omicron-fuelled surge in infections, saying society must "live with" the virus.

The Omicron variant has caused case numbers to soar in Norway, but hospital admissions for severely ill Covid patients have not increased in a population with wide vaccine coverage.

"We have finally reached the point where we can lift lots of the health measures we have had to live with this winter," Prime Minister Jonas Gahr Store told a press conference.

"We are going to have to live with a high level of infections -- we can live with a high level of infections," he added.

Norwegians will not have to quarantine if they are a contact of an infected person, although daily testing is recommended for five days, and the isolation period for Covid cases will fall from six to four days.

Remote working will no longer be obligatory and an unlimited number of people can visit other households and attend sporting events.

Travellers entering the Scandinavian nation will no longer have to undergo testing.

Restrictions on alcohol sales in bars and restaurants will also end.

The measures come into effect at 11 pm (2200 GMT) Tuesday.

Masks will remain mandatory in settings like public transport and shops where it is impossible to follow the recommended social distancing guidelines of one metre.

Norway will therefore not go as far as neighbouring Denmark, which on Tuesday became the first European Union country to remove all of its domestic Covid curbs.

More than 781,000 cases and 1,440 virus-related deaths have been recorded in Norway, where almost 91 percent of the population is fully vaccinated.

Norway's public health institute estimates that three to four million people from a population of 5.4 million could have been infected by this summer.