The Health Service Executive is urging everyone over 65, pregnant woman, people with chronic illnesses and healthcare workers to get vaccinated against influenza.
Over 56% of people over 65 with a medical card were immunised against flu in 2011, down from 60% the year before and significantly below the World Health Organisation's target of 75%.
Launching its annual flu vaccination campaign, the HSE stressed that the vaccine is the best defence against what can be a very serious illness.
The vaccine is free for at-risk groups, while people with a medical or GP visit card can also get the vaccine and their consultation for free.
Healthcare workers are also being urged to get vaccinated to protect themselves and to prevent spreading the flu to vulnerable patients.
The HSE has reassured potential recipients that the vaccine cannot give them the flu as it does not contain any live flu virus.
Pharmacies can administer the free flu vaccine to anyone over 18 in the at-risk groups.
Last year pharmacies were restricted to offering the vaccine to over 65s.
Speaking on RTÉ's News at One, Head of the HSE's National Immunisation Office Dr Brenda Corcoran said it is part of the HSE's efforts to encourage more people to get vaccinated this flu season.