NPHET is worried about the 'unusually high' number of coronavirus cases in recent days, Professor Philip Nolan says.
He said it's vital to 'sound a warning' to the public so the progress made in tackling the virus is not lost.
However, he said there have only been six 'very small' outbreaks reported in schools since they started reopening last week.
Yesterday saw 592 new cases confirmed, while 631 were reported on Wednesday.
These numbers are both above the five-day moving average of 499 cases.
It comes as schools continue to reopen and more non-COVID healthcare resumes.
Professor Nolan - chair of the NPHET modelling group - told Newstalk Breakfast the country has made 'exceptional' progress.
However, he said the situation remains 'very precarious'.
He said: “I want to emphasise the fact we have made enormous progress - we’ve gone from 6,500 to 500 cases a day… from 2,000 people in hospital with COVID-19, to 360.
“But I think we’re in danger of forgetting that that level of disease is twice what we saw in December and 50 times what we had as we entered the summer last year."
Professor Nolan said the current situation is 'rapidly evolving', but up until Monday there had been a 'slow and steady' suppression of the virus.
He said: “Frankly, we’re worried - it’s only in the last week that we’ve taken the first step in getting back to some of our key priorities.
“We were worried as we started that that we might see increased social mixing, and increased return to the workplace on the margins of that cautious reopening.
“A few days of unusually high case counts compared with preceding weeks… the fact we can see more social mixing and mobility in society… it’s a responsibility now to sound a warning to the public."
Schools reopening
Professor Nolan said there's no evidence that the reopening of schools is responsible for the recent increases.
He said: “Even though it’s early days, you see no particular change in the transmission of the disease in young people.
“We also know our public health colleagues are exceptionally careful around any incidents in schools.
"We only have six very small outbreaks declared in the last week - fewer than 10 cases linked to them at the moment.”
Meanwhile, Professor Nolan also said it's 'understandable' that some people are returning to the office or making more social visits to other people's homes after many weeks of tough restrictions.
However, he warned that people 'slipping slightly' can make a huge difference.
He said: "What we need to remember is that each of our individual small risk-taking behaviours adds up to a huge risk across society as a whole.
"I wouldn’t fixate on the number of 200 [cases]. The issue we need is to keep the reproduction number below one, and keep case numbers going downwards."
Despite the concerning signs, Professor Nolan said the vaccine programme is offering the country 'huge hope'.
He added it's now only a matter of weeks before older and vulnerable people will be protected.
