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The World Health Organization (WHO) says it is not yet clear whether Omicron, first detected in southern Africa, is more transmissible than other variants, or if it causes more severe disease.
"Preliminary data suggests that there are increasing rates of hospitalization in South Africa, but this may be due to increasing overall numbers of people becoming infected, rather than a result of specific infection," WHO said on Sunday.
Understanding the level of severity of Omicron "will take days to several weeks", it said.
The detection of Omicron has triggered global alarm as governments around the world scrambled to impose new travel restriction and markets sold-off, fearing the variant could resist vaccinations and upend a nascent economic reopening after a two-year global pandemic.
In its statement, the WHO said it was working with technical experts to understand the potential impact of the variant on existing countermeasures against Covid-19, including vaccines.
Canada on Sunday reported that two people who had recently travelled to Nigeria had tested positive for the variant.
Meanwhile, France said it had detected eight possible cases.
Britain said it will convene an urgent meeting of G7 health ministers on 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.
Omicron, dubbed a "variant of concern" last week by the WHO that is potentially more contagious than previous variants, has now been detected in Australia, Belgium, Botswana, Britain, Canada, Denmark, Germany, Hong Kong, Israel, Italy, the Netherlands and South Africa.
Many countries have imposed travel bans or curbs on southern Africa to try to stem the spread. Financial markets dived on Friday, and oil prices tumbled.
A South African doctor who was one of the first to suspect a different coronavirus strain said on Sunday that symptoms of Omicron were so far mild and could be treated at home.
Dr Angelique Coetzee, chair of South African Medical Association, told Reuters that unlike with Delta, so far patients have not reported loss of smell or taste and there has been no major drop in oxygen levels with the new variant.
In the most far-reaching effort to keep the variant at bay, Israel announced late on Saturday it would ban the entry of all foreigners and reintroduce counter-terrorism phone-tracking technology to contain the spread of the variant.
Prime Minister Naftali Bennett said the ban, pending government approval, would last 14 days. Officials hope that within that period there will be more information on how effective vaccines are against Omicron.
The top US infectious disease official, Dr Anthony Fauci, said Americans should be prepared to fight the spread of the new variant, but that it was not yet clear what measure such as mandates or lockdowns would be needed. He 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 on Sunday he expected to receive advice imminently on whether the government can broaden a programme of providing booster shots to fully vaccinated people, to try to weaken the impact of the variant.
More countries announced new travel curbs on southern African nations on Sunday, including Indonesia and Saudi Arabia.
South Africa has denounced the measures as unfair and potentially harmful to its economy, saying it is being punished for its scientific ability to identify coronavirus variants early.
South Africa's President Cyril Ramaphosa said on Sunday that his government was considering imposing compulsory COVID-19 shots for people in certain places and activities, and he slammed rich Western countries for their knee-jerk imposition of travel bans.
"The prohibition of travel is not informed by science, nor will it be effective in preventing the spread of this variant," Ramaphosa said. "The only thing (it) ... will do is to further damage the economies of the affected countries and undermine their ability to respond to ... the pandemic."
Omicron has emerged as many countries in Europe are already battling a surge in Covid-19 infections, with some reintroducing restrictions on social activity to try to stop the spread.
The new variant has also 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 shot, according to medical and human rights groups.