The HSE has urged children and teenagers to get a free nasal spray flu vaccine before Christmas.
The vaccine is on offer to all aged two to 17, with those aged 13-17 included for the first time.
It is available from participating GP practices and pharmacies, while children in senior infants and all children in primary age special schools can get their vaccine at school.
The HSE said an increase is flu and winter viruses is expected.
Dr Aparna Keegan, Consultant in Public Health Medicine at the HSE National Immunisation Office, said: "In the coming weeks and into the seasonal break we anticipate that flu, among other winter viruses, will begin to circulate more widely as we see an increase in socialising with friends and family over Christmas.
"In response to this, we are extending the age limit for the children's flu vaccine to include all those aged 13 to 17 years.
"We know children are twice as likely as adults to catch and spread flu and this poses a threat to them and others. Young children, in particular, are more likely than adults to be very sick from flu.
"Vaccinating children protects them – and their siblings, parents, grandparents and those who are vulnerable."
Children are given the nasal vaccine by spraying the vaccine once into each nostril.
Many of them report that it tickles when being given, the HSE said.
Speaking on RTÉ's Morning Ireland, Dr Keegan said last year was quite a bad flu season with "nearly 1,200 kids under the age of 15 admitted to hospital with flu".
"Flu is a really serious illness for kids as well," Dr Keegan said.
"So, it's important to remember that flu can have quite a detrimental impact. And when we looked at the data over a 10-year period, unfortunately 41 children succumb to flu as well.
"There could be quite serious complications for flu, so it's important that children get the flu vaccine to protect themselves primarily. But there's also an additional benefit that it can offer that wider protection to other people around them as well who may be vulnerable."
On the low uptake of the flu vaccine, Dr Keegan said there is a misconception that flu is a mild illness, "but actually in children, particularly younger children and children with underlying health conditions, it can have quite serious consequences".
