People in the Republic of Ireland have been advised to limit their contacts to those they will see on Christmas Day.
Ireland’s chief medical officer Dr Tony Holohan said Omicron now accounts for about two-thirds of the country’s Covid-19 cases.
Tighter Covid restrictions to curb the spread came into force in the Republic on Monday.
Hospitality and cultural venues including pubs, restaurants, cinemas and theatres must now close by 20:00.
The new rules are expected to remain until at least 30 January.
Dr Holohan said: “This week, try and only meet with the people with whom you will spend Christmas Day. Keep your contacts as low as possible in order to protect those around you.”
He added that there were reports of “a higher level of infection among household close contacts as a result of Omicron infection”.
“If there is one positive test, whether from a PCR or an antigen test, within a group such as a household then there is a significant chance that others are already infected, even if not yet testing positive,” he said.
“If one person from a household tests positive prior to an event or gathering, then none should attend.”
The Northern Ireland Executive will meet on Wednesday to discuss which restrictions could be re-introduced to tackle a rise in cases of the Omicron variant.
Scotland and Wales have already announced new measures to start after Christmas.