It's "more or less inevitable" that unvaccinated people will get infected with COVID-19, the HSE's Dr Colm Henry has said.
He suggested the Delta variant means people are unlikely to be able to "hide behind the vaccinated majority".
However, he said the impact will depend on how quickly unvaccinated people are infected.
The HSE's Chief Clinical Officer was speaking after a spike in case numbers in recent days.
The number of people in hospital with COVID-19 has also increased to 400 for the first time since the beginning of March.
Admissions to ICU have also increased by 20% in the space of a week, with latest figures showing 74 are receiving intensive care.
Dr Colm Henry told The Pat Kenny Show some of the recent increases in hospitalisations are likely down to a 'lag effect' from infections in recent weeks.
He said there's also still a significant number of unvaccinated adults out there, despite over 92% of people over 18 now being fully vaccinated.
He said: “There’s still probably between 40-50,000 people out there over the age of 40 unvaccinated. Those people remain highly vulnerable to COVID-19 - they will get it because it’s highly transmissible.
"The question is how quickly they will get it… over what timeframe. It’s not just the harm to themselves, but the pressure it will put on the health system.
“Given the transmissibility of this virus, it is more or less inevitable they will become infected given the Delta variant and the way it behaves.”
With no vaccines yet approved for younger children, Dr Henry suggested there's no evidence at the moment that those younger age groups are a significant source of infection or outbreaks.
"The virus is so transmissible it will find people"
Dr Henry said not all the current COVID-19 figures are as pessimistic as they may seem - saying there's been a significant weakening of the link between case numbers and the serious harm from the virus.
There were 318 deaths between April and October - compared to almost 1,500 in January alone. Around a third of were so-called "breakthrough infections", with Dr Henry saying a “significant majority” were among people with underlying conditions.
Dr Henry said “of course” there are some people who can’t get vaccinated, but he can’t understand why someone working in healthcare workers in particular “would in some way be anti-vax or consider vaccinations anything other than a hugely positive development in healthcare throughout the world.”
He said the HSE's in "constant discussion" with NIAC about booster doses, although they don't believe the evidence is currently there to offer healthcare workers an additional dose yet.
With the final stage of reopening now just over a week away on October 22nd, the HSE is expecting the current level of cases and hospitalisations to "rumble on" for the next few weeks.
Dr Henry observed: “So much depends on people’s behaviour. The virus hasn’t gone away - yes the vaccine affords huge protection, but it’s not 100% protection.
“Unless there’s a really good reason, I wouldn’t see why anyone wouldn’t [get vaccinated] - for their own protection.
“If anybody feels they can hide behind the vaccinated majority, I don’t believe that’s the case - I think the virus is so transmissible it will find people.”