The omicron variant of the coronavirus is likely to spread internationally, posing a "very high" global risk of infection surges that could have "severe consequences" in some areas, the World Health Organization (WHO) said Monday.

The U.N. agency urged its 194 member states to accelerate vaccination of high-priority groups and, in anticipation of increased case numbers, to "ensure mitigation plans are in place" to maintain essential health services.

"Omicron has an unprecedented number of spike mutations, some of which are concerning for their potential impact on the trajectory of the pandemic," the WHO said.

"The overall global risk related to the new variant … is assessed as very high."

To date, no deaths linked to omicron had been reported, though further research was needed to assess its s potential to escape protection against immunity induced by vaccines and previous infections, the WHO said.

"Increasing cases, regardless of a change in severity, may pose overwhelming demands on health care systems and may lead to increased morbidity and mortality. The impact on vulnerable populations would be substantial, particularly in countries with low vaccination coverage," it added.

The variant was first reported to WHO on Nov. 24 from South Africa, where infections have risen steeply.

A top South African infectious disease expert said Monday that existing COVID-19 vaccines are probably effective at preventing severe disease and hospitalization from the variant.

Professor Salim Abdool Karim added at a news conference that it was too early to say whether omicron led to more severe clinical symptoms than previous variants, although it does appear more transmissible and more likely to infect people who have immunity from vaccination or prior infection.

Karim said the variant is likely to fuel a surge in South Africa's coronavirus cases that could see daily infections treble this week. Health monitors reported over 2,800 infections on Sunday, up from a daily average of 500 in the previous week and 275 the week before.

"We can expect that higher transmissibility is likely and so we are going to get more cases quickly," Karim said at an online health ministry press briefing. "I am expecting we will top over 10,000 cases by the end of the week per day (and) pressure on hospitals within the next two, three weeks."

The variant has spread around the world, with new cases found in the Netherlands, Denmark and Australia even as more countries imposed travel restrictions to try to seal themselves off. Japan said on Monday it would close its borders to foreigners, joining Israel in taking the toughest measures.

The WHO, in its latest guidance, reiterated that countries should use a "risk-based approach to adjust international travel measures in a timely manner." Further advice would be forthcoming, it said.

"The presence of multiple mutations of the spike protein in the receptor-binding domain suggests that omicron may have a high likelihood of immune escape from antibody-mediated protection. However, immune escape potential from cell-mediated immunity is more difficult to predict," it said.

"Overall, there are considerable uncertainties in the magnitude of immune escape potential of omicron."

More data was expected in coming weeks.

"COVID-19 cases and infections are expected in vaccinated persons, albeit in a small and predictable proportion", it added.

The detection of omicron triggered global alarm as governments around the world scrambled to impose new travel restrictions and markets tumbled, fearing the variant could resist vaccinations and upend a nascent economic reopening after a two-year global pandemic.

Britain said it would convene an urgent meeting of Group of Seven nations' health ministers later Monday to discuss the developments.

Dutch health authorities said 13 cases of the variant were found among people on two flights that arrived in Amsterdam from South Africa on Friday.

Authorities had tested all of the more than 600 passengers on those two flights and had found 61 coronavirus cases, going on to test those for the new variant.

"This could possibly be the tip of the iceberg," Health Minister Hugo de Jonge told reporters in Rotterdam on Sunday.

Dutch military police said they arrested a married couple who left a hotel where they were in quarantine after testing positive for COVID-19, and were attempting to flee the country.

Omicron was dubbed a "variant of concern" last week by the WHO. On Sunday, Japan’s National Institute of Infectious Diseases raised its variant alert on omicron to the highest level. Japan has not reported any cases of the new variant.

Also Sunday, the top U.S. infectious disease official, Dr. Anthony Fauci, told President Joe Biden that it will take about two weeks to have more definitive information about the transmissibility and other characteristics of omicron, the White House said in a statement, adding that Fauci believes existing vaccines "are likely to provide a degree of protection against severe cases of COVID."

Fauci has said the variant is likely already in the country, although no cases have been confirmed.

In Britain, the government has announced measures, including stricter testing rules for people arriving in the country and requiring mask-wearing in some settings.

British health minister Sajid Javid said Sunday he expected to receive advice imminently on whether the government can broaden a program of providing booster shots to fully vaccinated people, to try to weaken the impact of the variant.

The new variant has thrown a spotlight on huge disparities in vaccination rates around the globe. Even as many developed countries are giving third-dose boosters, less than 7% of people in poorer countries have received their first COVID-19 shot, according to medical and human rights groups.

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