It would mean case numbers could be back up to 600 per day by the second week in January.
Public Health Officials believe the 'R' number is now close to 1, with people urged to limit their contacts as much as possible over the festive season.
NPHET's Head of Modelling Professor Philip Nolan says they've looked at what would happen if the 'R' number hit 2 during Christmas week.
"As you would imagine what that does is very significantly accelerate the spread of the virus through the core festive period, and leaves us in very rapid exponential growth in early January. And those scenarios, with a very high level of viral transmission during the Christmas period, would see something between eight and 800 and 1200 cases a day by mid January."
