Flu cases are expected to peak this week, but it will not immediately result in decreasing pressures on hospitals, the HSE's Chief Clinical Officer has said.
Hospitals around the country are experiencing a significant surge in activity.
Dr Colm Henry said there are currently 758 people in hospital with flu, but the number of cases in the community are still at a very high level at more than 3,000.
He said the uptake of the flu vaccine this year has been disappointing, particularly among children and healthcare workers.
Dr Henry said that around 43% of those hospitalised are over 65, but nearly 18% are aged under five.
Speaking on RTÉ's Today with Claire Byrne, he said that vaccines are never 100% effective but they are essential as part of a multi-layered approach to infection prevention.
This also includes hand washing and staying away from others when sick, Dr Henry said.
He explained that this season's flu - the AH1 variant - has not been dominant since around 2018/19 which might explain the surge in cases.
Dr Henry said many hospitals will not schedule non timed critical procedures around the Christmas period because this is when flu cases tend to surge.
However, he said all time critical procedures are not suspended during this time.
Dr Henry said the HSE is putting a huge focus on infection prevention and control this season as well as vaccines.
Outbreak management is very important and in the week leading up to last Saturday there were around 80 outbreaks in healthcare facilities.
Many of these were in acute hospitals and nursing homes but most of them remained small and self contained, he said.
Meanwhile, the Irish Nurses and Midwives Organisation said there were 700 admitted patients waiting for a hospital bed this morning.
It said that 444 patients are waiting in emergency departments, while 256 are in wards elsewhere in hospitals.
The hospital worst affected is University Hospital Limerick, with 102 patients waiting for a bed, while at Cork University Hospital 70 admitted patients are waiting, with 58 at University Hospital Galway.
