A new threat assessment from the European Centre for Disease Control said there are uncertainties around Omicron in terms of transmissibility, severity, and immune escape potential, though preliminary data suggests a substantial advantage over the Delta variant.

Based on mathematical modelling conducted by ECDC, there are indications that Omicron could account for over half of all Covid-19 infections in the EU/EEA within the next few months.

Taoiseach Micheál Martin said he spoke to EU Commission President Ursula von der Leyen last evening.

Ms von der Leyen told him that it would take another two weeks to gain a comprehensive understanding of the Omicron variant in terms of how infectious and virulent it is and to what extent it might be able to escape vaccines.

She also emphasised that vaccines would play a key role in protecting people, especially in the context of the Omicron variant.

Meanwhile, the Omicron variant is rapidly becoming the dominant strain of Covid-19 in South Africa less than four weeks after it was first detected there.

Cases have risen from a weekly average of 300 a day a fortnight ago to 3,500 a day.

The number of new cases reported in South Africa doubled from Tuesday to yesterday.

South Africa's National Institute for Communicable Diseases (NICD) said early epidemiological data suggested that Omicron was able to evade some immunity, but existing vaccines should still protect against severe disease and death.

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It said 74% of all the virus genomes it had sequenced last month had been of the new variant, which was first found in a sample taken on 8 November in Gauteng, South Africa's most populous province.

World Health Organization epidemiologist Maria van Kerkhove said that data on how contagious Omicron was should be available "within days."

The WHO is deploying a surge team to South Africa's Gauteng province, epicentre of the outbreak, to help with surveillance and contact tracing.

The organisation's Regional Emergency Director for Africa, Salam Gueye, also said the WHO was providing technical assistance to boost the production and distribution of medical oxygen in Botswana, where Omicron has also been detected.

South Africa was the first country to detect the variant, prompting many countries including Ireland to impose travel restrictions on countries in southern Africa.

United Nations Secretary-General Antonio Guterres hit out at what he called "travel apartheid".

He said testing should be used instead of travel restrictions.

"Blanket travel bans will not prevent the international spread and they place a heavy burden on lives and livelihoods," the WHO said, while advising those who were unwell, at risk, or aged 60 and over and unvaccinated to postpone travel.

Meanwhile, GlaxoSmithKline has said that a pre-clinical analysis of an antibody-based Covid-19 therapy it is developing with US partner Vir has indicated that the drug works against the new Omicron variant.

Further tests will be carried out on the sotrovimab therapy to firm up the results, GSK said in a statement.