The Health Service Executive did not offer an enhanced influenza vaccine for people aged 65 years and older this season as it was not deemed cost-effective.
Instead, a standard vaccine is being offered.
Enhanced flu vaccines include adjuvanted vaccines and high-dose flu vaccines.
Adjuvanted vaccines include an extra ingredient called an 'adjuvant', that increases the immune response, compared with standard vaccines.
High-dose flu vaccines, which contain a stronger dose of the ingredients that cause the immune response, are around four times higher than standard vaccines.
A Health Information and Quality Authority (HIQA) report last year estimated that switching to a adjuvanted vaccine for people 65 years and older, rather than the standard flu vaccine, may be the best use of resources.
However, the best value option would depend on the price the HSE would need to pay for each type of vaccine and these prices are confidential.
It found that while the vaccine would likely cost more, this approach would be more effective and overall would cost the HSE less each year, because fewer people would require hospitalisation compared with using the standard flu vaccine.
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Hospitalisation levels for influenza have risen in recent weeks as a likely severe flu season has started early.
So far this season, there have been nine deaths, four in those under 65 years and five in people over 65 years of age, according to the latest Health Protection Surveillance Centre figures.
In a statement to RTÉ News, the Department of Health said the HSE was asked to have a tender process with the drug firms offering the enhanced flu vaccine, to determine if these vaccines could be bought at a price that was cost-effective.
"The HSE advised the Department that neither of the enhanced vaccines for older people met the criteria set out in the Health Technology Assessment for net monetary benefit compared to standard flu vaccine," the department said.
The HIQA review last year said that replacing the standard flu vaccine with a high-dose vaccine was estimated to be more effective.
It also said that ten countries offer an enhanced flu vaccine, free of charge to some, or all of this age group.
But it said this approach would likely cost the HSE more money overall, as the expected higher cost of this vaccine type would only partially be offset by cost savings.
It added that the best value option would depend on the price the HSE would need to pay for each type of vaccine and that these prices are confidential.
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The National Influenza Vaccination Programme provides a free flu vaccine to those most vulnerable to the effects of the flu.
The Department of Health said that the HIQA Health Technology Assessment noted that whether changing to offering an enhanced flu vaccine to those aged 65 and older would be a good use of public funding depended on the price of the vaccine.
The Department added that given the healthcare budget is finite, and decisions regarding increased spending relating to a change in one area could impact the provision of other health technologies and treatments within the healthcare system, the cost-effectiveness must be considered in any decision-making process.
The HSE said that the selection of flu vaccines for the HSE programme is informed by recommendations from National Immunisation Advisory Committee (NIAC), a public tendering process to the drug firms, HIQA cost-effectiveness analyses and Department of Health policy.
It said that the influenza vaccine used each season is selected in line with international guidance issued annually by the World Health Organization (WHO).
The HSE said that in advance of each influenza season, the WHO reviews global surveillance data and recommends the specific influenza strains that manufacturers should include in that season's vaccine based on the flu strains that are considered most likely to circulate.
It said that all licensed influenza vaccines for the 2025/2026 season comply with WHO recommendations.
For adults, trivalent influenza vaccines are recommended by the WHO and by the National Immunisation Advisory Committee.
The HSE said it implements the influenza vaccination programme in line with NIAC advice and Department of Health policy.
It has also advised that there is still time for people to avail of the influenza vaccine, including health staff.
Flu will peak around Christmas week
The HSE's Chief Clinical Officer, Dr Colm Henry, said there were "over 1,600" flu presentations to emergency departments in the week up to midnight on Saturday, compared to just over a thousand the week before that.
Speaking to RTÉ's News at One, he said 657 of those were admitted to hospital, compared to 415 the week before.
"What we're seeing there right across the spectrum of community and hospitals is a three-fold increase in impact in presentations in emergency departments and admissions over a two week period," he said.
"We predict that we're going to see a peak sometime around Christmas week and perhaps a plateau for a number of weeks after. Based on our experience last year, that plateau will have an ongoing impact on healthcare for a few weeks into January or possible until the end of January.
"The impact of this particular variant in terms of severe illness is particularly in the extremes of age, so 40% of our hospitalisations occur in those over 65. But it's important to note also that 40% are in those less than 15 years of age.
"We're not innocent bystanders watching helplessly as the figures go up, there's still a lot we can do. The most effective intervention we know is that of the vaccine. The vaccine is very effective at preventing serious illness among vulnerable groups."
Dr Henry said it is part of normal operations to curb elective activity to a certain degree during December and January, and to make up for it later in the year.
He said all urgent and emergency procedures are going ahead, while most sites are carrying out elective activity.
"But we can expect to see a curb on what we call elective and non urgent activity in the coming weeks in response to the demands on the acute and unscheduled care system," he added.
When asked if it was a mistake not to offer an enhanced influenza vaccine for people aged 65 years, Dr Henry said "not at all".
He said the vaccine on offer is "highly effective" and the vaccine programme for older people is "among the most effective in Europe" with a 75% uptake last year among those aged 65 and older.
"As with any drugs we approve or any vaccines we approve, we have to look at both the suggested additional effectiveness and the additional cost, and based on assessment that was made through what we call the health technology assessment, whatever additional benefits were presented in relation to this enhanced vaccine were not justified by the considerably increased costs," he said.
"It is important for people to understand - the vaccine that we are giving is effective and can prevent serious illness, particularly in vulnerable groups."
