The Covid-19 booster vaccination is to be offered to anyone aged 18-49, the Health Service Executive has confirmed.
It comes as the country's hospitals remain under severe pressure with significant numbers of people waiting for admission on trolleys in emergency departments.
The overcrowding is being driven by a rise in respiratory infections, including Covid and the flu.
The number of patients in hospital today with Covid-19 is 723, down 14 on yesterday's figure.
Of these patients, 38 are in ICU, a rise of six on yesterday's number.
A week ago, there were 655 patients with the virus in hospital.
The HSE has said anyone aged 18-49 can make an appointment for their second Covid-19 booster dose through the HSE website, with appointments available from today.
In a statement, the Department of Health said the minister had also authorised a first vaccine for infants and children aged six months to four years following advice from the National Immunisation Advisory Committee (NIAC).
It added: "The relevant Statutory Instrument and operational plans are currently being prepared and the HSE will announce early in 2023 when this cohort are being called forward for vaccination."
A "bivalent booster is now authorised for already eligible children aged 5-11 years", it said.
"The use of Comirnaty Original/Omicron BA.4-5 bivalent vaccine as a booster has now been approved for those aged 5-11 years who are currently eligible for a booster vaccination (i.e. those with immunocompromise associated with a sub optimal response to vaccines at the time of their primary or additional vaccination)."
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Bivalent vaccine
The national lead of the Covid-19 vaccination programme said 18-49 year olds will receive the bivalent vaccine.
Speaking to RTÉ's News at One, Eileen Whelan said people are encouraged to go to the central vaccination clinics, while the booster vaccine will also be available from GPs and pharmacies.
"The bivalent vaccine offers broader protection and it is going to offer protection to the new variants which are in circulation," she said.
Referring to approval for a first vaccine for infants and children aged six months to four years, Ms Whelan said: "We do know that the highest risk is in the older age groups but NIAC have assessed the clinical evidence and they have assessed the safety evidence both in other countries and the evidence in relation to the 5-11 year olds and the evidence that currently exists in administering bivalent and they have assessed this as safe and as the right thing to do."
Ms Whelan stated that a number of measures will have to be put in place before the rollout of vaccines to this younger age group begins.
She also confirmed that children aged 5-11 years who are immuno-compromised are now being offered a booster, which is also the bivalent vaccine.
She stated that while a Covid-19 vaccine will not prevent transmission of the virus, its impact will be curtailed.
"The evidence is there that if you receive a vaccine, the severity of your illness will be less and that's the evidence," she said.
Take up
The HSE's Chief Clinical Officer, Dr Colm Henry, said uptake of the second booster vaccine was not as widespread as the HSE had hoped.
Speaking on RTÉ's Morning Ireland, Dr Henry said uptake of the first booster was strong at 77%, but figures began to wane because "the fear factor wasn't there as much".
"We're hopeful that people will take this up to protect themselves and anyone vulnerable they're in contact with, and in a broader sense to protect the health service," he said.
This booster can prevent infection as well as serious illness, although that protection begins to wane after a number of months, he said.
Dr Henry also said the situation facing hospitals around the country is of "significant concern".
"The concern arises from the upward trajectory of flu and Covid - particularly flu. We saw a 100% rise in lab confirmed cases of the flu last week compared to the week before," he said.
"That's about 2,300 cases of flu, compared to half that the week before."
Dr Henry said there was a similar rise in the number of hospitalisations, and there is "no sign yet" that they have reached the peak.
There has been a rise in over-75s presenting to emergency departments, he said, with half of them being admitted.
The HSE is investing in "hubs of care" for older people and chronic disease, which will employ 2,000 people, he added.
Meanwhile, Paul Moynagh, Professor of Immunology at Maynooth University, said that while targeting the more vulnerable groups with second boosters makes sense, there is limited information on the effectiveness of the rollout of second boosters for younger people.
"The people who have been protected by the second boosters are the vulnerable groups whereas younger people are still benefitting from the protective effects against serious disease," he said.
Speaking to RTÉ's News at One he also stated that he is not convinced that more people being vaccinated will ease pressure on hospitals.
"If we are trying to use vaccination to protect our health system, yes it will protect people from getting seriously ill," he said.
"In terms of protecting and chasing variants by new vaccines, I don't think that approach is going to work."
