The HSE is warning that a number of people have already died as swine flu returns to Ireland.
The health service said over 200 people have been diagnosed with the strain - known as H1N1 - in recent weeks.
Dr Kevin Kelleher, assistant national director of public health at the HSE, said the best way to protect against all forms of flu is to get the vaccine.
He said children and the elderly are most at risk.
"It then has this particular predilection to cause severe respiratory disease in a number of people who are under 65 with pre-existing disease and pregnant women," Dr Kelleher said.
"Unfortunately it will just do it randomly in totally well people as well.
Those people will end up in ICU and unfortunately, a number will die.
The HSE is urging all people in at-risk groups for the flu to get vaccinated if they have not done so already.
Flu levels are likely to increase over the coming weeks when flu begins to actively circulate in the community.
"Flu is now beginning to circulate, and as schools reopen and people return to work we are likely to see increased levels," Dr Kelleher said.
The predominant strain currently circulating is H1N1, which is affecting younger age groups more than strains that have circulated in previous years.
"This year’s vaccine is a good match for circulating strains and it is not too late to get the vaccine."
People with symptoms should stay at home and children should not attend school if showing signs of flu.
