The majority of countries in Europe are reporting an increase in the rate of flu cases, according to the World Health Organization's Team Lead for Communicable Disease has said.
Dr Marc-Alain Widdowson said the increase in flu cases has come around four weeks earlier than it normally would.
Speaking on RTÉ’s Today with David McCullagh, he said the number of cases in Ireland is "quite high".
Ireland is one of six countries - along with Kyrgyzstan, Montenegro, Serbia, Slovenia and the United Kingdom - which have recorded an over 50% positive rate for flu in those tested with symptoms.
Dr Widdowson said this year’s strain of flu has mutated in a "drifting process" and has "escaped immunity".
"This happens almost every year, that flu strains will drift a little bit. This particular time, it has drifted a bit more than we would have wanted," he said.
Vaccine 'working well' against current strain
However, Dr Widdowson said that research from Ireland showed the vaccine is "working well" against the current strain.
He said there is no evidence that the strain is causing more severe disease.
Dr Widdowson advised people to get a vaccine, to stay at home if they have symptoms or wear a mask while outside and wash their hands.
"One of the things we are a little worried about is, because this flu season is coming earlier and with Christmas coming with the holidays, lots of grandchildren will be visiting their grandparents, some of which are frail and some of which are in nursing home facilities.
"And it is possible the peak of this particular virus season will hit us just in that festive season," he said.
He urged people to be "particularly careful" this year.
Dr Widdowson said there is concern over misinformation and disinformation about vaccines.
He advised healthcare workers to get vaccinated to protect themselves and their patients.
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