The Department of Health has confirmed 6,307 new cases of Covid-19.
There are 429 people in hospital who have tested positive for the virus, down 14 since yesterday. Of these 100 are in ICU, a reduction of two.
There has been a total of 5,890 coronavirus-related deaths notified in Ireland. This includes 55 deaths newly notified in the past week.
In Northern Ireland, 3,231 new infections have been reported, along with three further deaths.
Chief Medical Officer Dr Tony Holohan said: "Our recent research has shown that many people have reviewed their plans for Christmas, reduced their social contacts and changed or postponed plans in order to protect themselves and their loved ones.
"This is a big sacrifice that many have made and will do much to protect all of us over the festive period."
He added: "It is important that we remember that Covid-19 will still be circulating at a high level after Christmas and into the New Year.
"Even though it is a difficult message to hear, we must all continue to keep our social contacts low and do all we can to continue to adhere to the public health advice."
Dr Holohan said young people are making "a really big effort to act responsibly and to follow the public health advice and I would like to thank them for their significant efforts".
He urged: "It is really important that we all remember that there is no one person or group or thing to blame in the situation we find ourselves in other, than the Covid-19 virus.
"Booster vaccination is accelerating across the country and the images we see of people queuing for their vaccine, prioritising that appointment before Christmas are heartening.
"Please stick with all of the measures that we know work as we move through the next few difficult weeks, including attending for vaccination."
Dr Holohan continued: "If you display any symptoms of Covid-19, however mild, then self-isolate and arrange a PCR test as soon as you can. This is the single most important piece of basic public health advice.
"Do not meet up with others and put them at risk of Covid-19 or another respiratory infection. Do not rely on a negative antigen test as a basis for not isolating."
Earlier, the Chief Medical Officer warned that Ireland is facing a very significant surge of Covid-19 infection, with a rapid rise in the infection rate among those aged from 16 to 34.
Dr Holohan said that over one million of those aged over 40 in eligible groups have not come forward for their booster vaccine.
Speaking on RTÉ's Today with Claire Byrne, the CMO said while rising infections are not confined to those aged 16-34, there is a dramatic rise in infections among this cohort that is consistent with what is being seen in other European countries.
Dr Holohan said one in six young people have yet to come forward for any vaccine and he urged them to do so.
He said there is "a substantial job to do" to issue a third vaccine to those over 40 who are at risk of severe infection and he asked them to come forward.
"Vaccines really work, particularly when boosted, in preventing infection from becoming severe infection and we expect that to be the case with Omicron.
"Don't put off the opportunity to be vaccinated if it comes your way," he added.
Meanwhile, Chief Executive of the HSE Paul Reid has revealed that yesterday was another record day in the vaccination and booster programme.
In total, 108,000 vaccines were administered in one day, including 103,000 boosters.
In a tweet, he said 186,000 vaccines have been administered in two days. So far 1.7 million people have received either a booster or a third dose.
Another record day in the vaccination & booster programme yesterday. 108,000 vaccines in total administered in one day (103,000 boosters). Thats 186,000 vaccines administered in just 2 days. 1.77M boosters & 3rd doses done. Thanks to all who are vaccinating & the public @HSELive
— Paul Reid (@paulreiddublin) December 22, 2021
Risk of re-infection with Omicron 'very high'
A professor of immunology at Dublin City University has described the decision to shorten the waiting time for a booster shot for those who have recovered from Covid-19 as "really good news".
Professor Christine Loscher said the risk of re-infection with Omicron is very high, around five times higher than it was with the Delta variant.
She said the booster shot is really important for everyone to have, regardless of whether they have contracted the disease.
Speaking on RTÉ's Morning Ireland, Prof Loscher said those people who had received the Janssen shot, particularly in the younger cohorts, need to receive a booster shot as a matter of priority.
She said these people should be able to attend a walk-in clinic or get a booster shot from a pharmacy.
Prof Loscher said she believes case numbers will increase steadily over the next few days, but the impact of the high amount of Omicron that is currently circulating probably will not be seen until later this month.
When asked about Israel's decision to give a fourth vaccine dose to certain groups, she said that there currently is not enough evidence to suggest that the recent booster shots will wane to such an extent that it would impact on the spread of Omicron in the near future.
