Trinity Professor Luke O'Neill says he 'can't see' schools reopening next Monday if coronavirus numbers continue to rise.
He said it's a very difficult decision, but the situation 'could be moving' in the direction of an extended closure if transmission is out of control.
It comes as the Education Minister is set to meet opposition parties later to brief them on the reopening of schools.
Norma Foley is being urged to clarify when schools will be able to allow students back into classrooms.
At present, schools are due to reopen on January 11th - but there are concerns that won't happen due to rapidly rising COVID-19 case numbers.
Ministers have said they still plan for students to return in a week's time, but the situation will be reviewed this week.
HSE Chief Executive Paul Reid, meanwhile, has warned that daily COVID-19 case numbers are likely to hit 7,000 over the next few days.
Speaking on The Pat Kenny Show, Professor O'Neill said everything will depend on what we see over the next couple of days.
He said “Closing the schools will have all kinds of knock-on effects, so it’s a very hard one.
“If we get to Wednesday, Thursday, Friday and the numbers are even higher… I can’t see the schools opening, because it’s another risk.”
He said the opening schools may not be a “huge risk”, but will need to be considered if the numbers are out of control.
Some of the recent debates on coronavirus vaccination have focused on whether there could be a longer gap between vaccine doses to make most efficient use of supply.
Professor O’Neill explained: “Let’s say in Ireland, you could use up the entire first supply in the hope that the second supply would come in and then you revaccinate at the 21 day point.
“The thing is… is that a risk? Maybe the second supply won’t come, and that means it won’t be as efficacious as you’d like - because it's no doubt the second shot boosts everything."
However, he noted evidence from the Oxford / AstraZeneca trials suggests the first dose is very effective - and officials could wait 12 weeks before administering a second dose to people.
The leading immunologist observed: “The Canadians are using up all their supply, some of the provinces… in the hope the second supply will come in on time. They’re saying that will decrease cases by 42% if they use up their first supply.
“It’s a hot debate at the moment.”
In terms of vaccination programmes, Professor O'Neill said Israel is currently ‘top of the league’ - with 12% of the population vaccinated already, and full vaccination expected by the end of March.
He said: “The question is why - why have the Israelis achieved what other countries haven’t?
“There’s a few reasons. They have a highly digitised health system… everybody is tracked from birth. They can find everyone… central government organises everything.
“They bought loads of vaccine supply very early - they probably paid above the [standard] price… they felt that was justified, for obvious reasons.”
Professor O’Neill said Israeli Prime Minister Benjamin Netanyahu also took ‘the bull by the horns’ - reportedly getting involved in the price negotiations, and also publicly getting vaccinated himself.
He also noted a lot of work was done to get ‘vaccine hesitant’ group - such as many within the Orthodox Jewish and Israeli-Arab communities - on board with the programme.