Professor Luke O'Neill says it's "still an unknown" why COVID-19 case numbers are climbing in Ireland.
The leading immunologist says society does need to fully reopen at some stage, but he "doesn't envy" the decision Cabinet has to make next week.
Under the reopening plan announced in August, almost all remaining COVID-19 restrictions are due to be lifted next Friday, October 22nd.
However, there has been growing concern among health officials about the rising COVID-19 case numbers and hospitalisations.
The five-day moving average is now more than 1,600 - an increase of over 500 compared to this time last week.
Both the Taoiseach and Tánaiste have now warned they can't guarantee all restrictions will be lifted, saying it will depend on the advice from NPHET next week.
Professor O'Neill - Professor of Biochemistry at Trinity College Dublin - told The Pat Kenny Show said there does appear to be growing concerns among officials.
He said: “The trend is not great, they say. We’re waiting for this final analysis… we haven’t seen some of those numbers. But it does look as though cases are climbing.
“The question is why this is happening? We’re number seven in Europe now in terms of the number of cases - that’s not a great place to be. We’ve got a lot more cases than, say, Denmark.
“The truth is we don’t know why these cases are climbing all the time - that’s still an unknown."
He said COVID-19 cases are not overwhelming hospitals, but there are some potential factors behind the recent spike.
He noted around 300,000 people in the over-12 age group remain unvaccinated, while a "big" reason is people not staying home when they have symptoms of the virus.
The immunologist said: “If you have symptoms, don’t go into work, don’t go into school: then it won’t spread so much."
However, he said there is cause for optimism, including no further major variant of concern having yet popped up - saying "that gives us a bit of hope that we mightn’t see an even worse one".
He said the booster campaign for older people and care home residents is also very important, as that should help counter any possible waning of immunity in those groups.
With case numbers still rising for now, however, Professor O'Neill believes a tough decision lies ahead for ministers next week.
He said: "We do need to open up at some point, but I don’t envy the Cabinet decision next week.
"If the numbers are really bad, what are they going to do? It’s a tricky question."