Professor Philip Nolan says Ireland is now 'clearly in a third wave' of the coronavirus pandemic.
Chair of NPHET Irish Epidemiological Modelling Advisory Group was speaking as there were 727 new cases of the virus confirmed here on Tuesday.
There were no new deaths recorded.
There have been a total of 80,267 cases and 2,158 deaths here.
Of the cases notified on Tuesday:
- 359 are men/366 are women
- 62% are under 45 years of age
- The median age is 36 years old
Some 311 of these cases are in Dublin, 51 in Kilkenny, 48 in Wexford, 44 in Donegal and 44 in Cork.
The remaining 299 are spread across 19 other counties.
As of 2.00pm, 241 COVID-19 patients are hospitalised - of which 29 are in ICU.
There have been 18 additional hospitalisations in the past 24 hours.
Prof Nolan said: "The virus is transmitting very rapidly, faster than we have seen at any point since March.
"The case numbers are growing at least 5% to 7% per day and, of particular concern, across all age groups."
Dr Tony Holohan, Chief Medical Officer, said: "Our current disease trends are gravely concerning.
"The situation has deteriorated rapidly in recent days.
"A total of 3,837 cases have been notified in the past seven days. The five day rolling average has increased from 339 on 17th December to 616 on 21st December, an 82% increase.
"It is now as important as it was back in March to limit your contacts and protect your loved ones."
While Dr Lorraine Nolan, chief executive of the Health Products Regulatory Authority, said: "Today we welcome a highly significant announcement from the European Medicines Agency in their recommendation to approve a first vaccine for COVID-19 in Europe.
"It is a key development in our continued efforts to tackling this pandemic.
"This will become one additional defence in our suite of public health actions to protect us from COVID-19."
And Dr Cillian De Gascun, director of the National Virus Reference Laboratory, said a new strain of the virus - which has seen a travel ban to and from the UK - has not yet been detected in Ireland.
But he added that the possibility is there: "To date, the novel SARS-CoV-2 UK variant has not been detected in Ireland.
"However, as we know the variant has been circulating in the UK since September, therefore we cannot exclude the possibility that the variant is already in Ireland.
"We are prioritising the sequencing of cases with confirmed or suspected links to the UK."