“We and other countries – the Netherlands Germany, Italy – have paused, and it is just a pause, the use of AstraZeneca,” she said.
“It is about a slight deferral to make sure that we are doing the right thing, using appropriately and giving it to the right people.”
Earlier Health Minister Stephen Donnelly said 30,000 people due to receive the AstraZeneca jab this week will have their vaccinations rescheduled in the next few weeks.
The European Medicines Agency (EMA) is expected to complete its safety review of issues relating to the jab later this week.
Professor Butler, chair of the National Immunisation Advisory Committee (NIAC), said it was not an easy decision to pause.
“This isn’t about just the normal clots that can occur, it was about a few, very particular and rare conditions occurring, clustering together, for example the three cases in Norway were in a single hospital within a two-week period, that just made one say, ‘we need to stop and think’,” she said.
“These things can happen by chance, so we just have to wait for the outcome (of the EMA investigation).
“It is a risk balance in everything and here, it seemed justified to make that pause until we got a sense of what the risk was. It might end up this is an absolutely infinitesimally small risk.
“Covid itself is a very real risk… what we’re just doing by this pause is getting to know, ‘is there a risk on the other end of it, and it is so small that this far outweighs it?”
Dr Ray Walley, of the National Covid-19 GP Liaison Committee, said the ongoing vaccination programme is “not affected by the AstraZeneca issue”.
“The majority of GPs are using Pfizer, with some using Moderna,” he said.
Monday saw the second day in a row of no deaths with coronavirus reported in Ireland.
However 575 new cases were notified by the Department of Health and on Monday morning, 360 Covid-positive patients remained in hospital, of whom 85 were in ICU.
Deputy chief medical officer Dr Ronan Glynn has urged caution, pointing to a “further wave” of coronavirus playing out across Europe.
He said 18-19 countries have been seeing a “deteriorating” picture over the last two weeks.
Dr Glynn also warned that while case numbers have fallen, Ireland is “by no means out of the woods on this”.
He said despite St Patrick’s Day and Easter approaching, people must “not drop their guard”.
“If we mix with other people over the next three weeks, given the levels of disease in the community, we will see a significant deterioration and none of us want to see that,” he said.
“We’re not trying to put the fear into people, we’re just asking people to protect themselves and their families because whilst vaccines are on the way, and they will make a very significant difference, they will not stop a wave over the coming weeks.”