People have been advised not to travel to seven countries in southern Africa unless "absolutely necessary" as a precautionary measure over a new Covid-19 variant.

The countries are Botswana, Eswatini, Lesotho, Mozambique, Namibia, South Africa and Zimbabwe.

Ireland currently has no direct flights running to any of these countries.

The World Health Organization has classified the new Covid-19 strain as a variant of concern.

The B.1.1.529 variant, which has now been given the designation Omicron, has a "very unusual constellation" of mutations, which are concerning because they could help it evade the body's immune response and make it more transmissible, South African scientists have said.

Belgium has detected Europe's first confirmed case of the variant, while EU states and Britain have agreed to temporarily suspend travel to southern Africa after the detection of the variant there.

Speaking on RTÉ's Six One News, Minister for Health Stephen Donnelly said residents of Ireland in the seven countries should return to Ireland as soon as possible and follow new public health protection measures upon arrival.

"I am concerned. The initial indications are that it is highly transmissible. In South Africa it has outperformed and taken over from Delta," Mr Donnelly said.

He said it was not yet known if the variant can evade vaccines. "We have to take this deadly seriously," he added.

Mr Donnelly said there were no direct flights from these countries into Ireland so restricting flights was not a measure that needed to be used.

Among the new measures, he said there will be a pre-flight PCR test regardless of vaccination status, a mandatory home quarantine involving two PCR tests for at least 10 days assuming the second test has a result of not detected.

Gardaí will play a role to play in policing this, he said.

Compliance will be easier to manage compared to earlier quarantines due to a smaller amount of people involved, he added.

Mr Donnelly said the Chief Medical Officer has advised Government to consider mandatory hotel quarantine.

Legislation around mandatory hotel quarantining has lapsed, and Mr Donnelly said legislation now needed to be prepared in advance of any decision on whether to re-introduce it.

The EU is considering travel advisories involving Belgium, Israel and Hong Kong, he said.

Travel advice for other countries will be kept under constant review.

Mr Donnelly said no case of Omicron had been detected in Ireland or the UK, but testers had not been looking for it in Ireland until now.

He said testing for Omicron and whole genome sequencing of cases linked to the seven countries will now begin.

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'Most worrying variant we have seen'

Professor of Experimental Immunology at TCD Kingston Mills said it is "the most worrying variant we have seen yet".

Speaking on RTÉ's Today with Claire Byrne, Prof Mills said: "I was shocked last night when I read the reports about it - really insidious mutations in the spike protein (that) not only mimic those in the Delta, but also the Beta variant - the original South African variance.

"That variant, people may not remember, caused the real problem for the AstraZeneca vaccine, so much that it only had 10% efficacy and South Africa stopped using it, so it has the worst of the Beta variant and the worst of the Delta variant in terms of transmissibility."

Prof Mills added: "We've got to keep this virus out of Europe at all costs, including Ireland, so we need to put in place very stringent measures to quarantine, isolate anybody that's come either directly or indirectly from South Africa or other countries where there is this variant."

Assistant Professor of Virology at Trinity College Dublin Kim Roberts said there is the potential that vaccines may be less effective against the new variant and one of the concerns around it is the exceptional number of mutations.

However, she said, it does not mean it is back to square one, as vaccines will still give a good degree of protection.

Dr Roberts said it is reassuring to know that scientists are "plugging into the changes" in their experiments to make sure PCR tests are working correctly and understanding how the virus is working.

Earlier, a Professor of Immunology at Dublin City University said there is not much information yet on the variant, but vaccine companies have been able to tweak vaccines to become more specific to target variants as they arise.

Speaking on RTÉ's Morning Ireland, Professor Christine Loscher said one of this new variant's mutations is similar to the one in Alpha and Delta, which might make it spread faster.