Colm Henry is hopeful Ireland's current surge in COVID-19 cases will drop back in the next few weeks.
The HSE's Chief Clinical Officer was speaking as latest figures show 812 patients in hospital have been diagnosed with coronavirus.
Around half of those are being treated for other medical issues and testing positive incidentally.
But over 300 confirmed hospitalised cases have never been vaccinated against COVID-19.
Colm Henry told Pat Kenny evidence suggests infections should drop in the next fortnight.
"It is a surge, a slower upward surge - we're drawing some hope from the experience in Portugal, where this plateaued for four to six weeks and then began to fall.
"We're about week three or four into that".
He says the surge is largely driven by two sub-variants of Omicron.
"This current surge [is] largely driven by these sub-variants BA.4 and BA.5, which have - as we call it - a growth advantage over the previous variants.
"They're more transmissible, or they evade the immunity we had from natural infection ot from vaccination".
He says unvaccinated people in hospital are disproportionately represented.
"If you're not vaccinated you've no protection against either infection, clearly, and you've no protection against serious illness.
"Look at the profile of patients hospitalised today: there are over 300 people in hospital today with COVID who are not vaccinated.
"They're totally disproportionately represented."
And he says boosters will be expanded when they get the green light from the National Immunisation Advisory Committee.
"I can't speak on behalf of NIAC - I do know some of the individuals involved, Karina Butler, people of enormous integrity who really want to do the right thing.
"Sometimes it seems people are frustrated at the pace of decision-making, but I know from regular engagement with her that she is on this seven days a week".