People are getting multiple booster vaccine appointments because of a glitch between the Health Service Executive's vaccination database and the IT systems used by pharmacies and GPs.
The HSE has confirmed that people are being issued with HSE booster appointments after they get booster vaccinations at their GPs or local pharmacy, because of the "gap" between the computer systems and databases.
The Irish Pharmacy Union (IPU) and Irish College of General Practitioners said the "gap" is caused by a time lag as long as two days between their information technology systems and the HSE's.
In a statement to RTÉ's Morning Ireland, the HSE said that because "GPs and pharmacies operate on different IT systems, there may be a gap in notification of the vaccination which can, from time to time, result in multiple appointments".
It added that because GPs, pharmacies and vaccination centres are rolling out vaccines, "people will occasionally get appointments from multiple sources or after they have already been vaccinated".
The HSE statement came after the issue was raised by pharmacists and GPs.
The HSE's Lead on Vaccination, Test and Trace Programme said everyone knew there would be challenges in the system, but that it was decided to push ahead with the three-stream approach because it gave people different options.
Speaking on RTÉ's Morning Ireland, Damien McCallion said people are being linked to vaccination centres as close to their home address as possible and efforts are also being made to be as flexible as possible to allow people change locations.
He said that vaccination centres will close on Christmas Day and St Stephen's Day, but will be open every other day over the Christmas period.
Mr McCallion added that the vaccine stock for children aged five to 11 is due to begin arriving from next week.
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The Irish Pharmacy Union said a mismatch between two vaccine databases - PharmaVax and CoVax - is resulting in 'no shows' at vaccination centres because vaccinated people are being issued with appointments days after getting booster jabs.
The glitch, the IPU said, is caused by a time lag between the point vaccine and patient data is sent from the PharmaVax system to the HSE's CoVax database.
"At the moment, if somebody gets their booster vaccine in a pharmacy we enter those details into the Pharmavax system, which is a HSE developed and owned system that feeds the national CoVax database," said IPU Secretary General Darragh O'Loughlin.
"The details are going in from the pharmacy as we are doing the vaccine but when they transfer over to that central CoVax database, it doesn't automatically trigger a cancellation of a booster appointment.
"What that means is that people are having a vaccine in a pharmacy - they are getting a booster, they are going home - and then a day or two later they might be issued an appointment in a vaccination centre by text and they are then struggling to cancel that appointment," added Mr O'Loughlin.
"As a result, these people turn up as a no show in a vaccination centre despite the fact that they've already had their booster."

Mr O'Loughlin added: "The system needs to be examined. It needs to be smoother.
"The information that comes in from pharmacies and from GPs - when they are doing the vaccination - needs to go in real time into the CoVax database so we are reading and writing that information in real time.
"Then it should automatically identify a person as someone who has already had their booster and cancel any appointment that's there and not issue any fresh ones."
The Irish College of General Practitioners agreed with the IPU. It confirmed there are similar time lags when GPs input vaccination information through their Helix and Socrates systems.
Clinical Lead on Covid-19 for the Irish College of General Practitioners Dr Nuala O'Connor said in a statement: "Participating GPs input vaccination data to the Socrates or Helix IT systems as soon as Covid-19 vaccinations are administered to patients.
"We understand this data can take up to 48 hours to be inputted to the Covax database."
The HSE said that because GPs and pharmacies operate on different IT systems there may be a gap in notification of vaccination, which may result in multiple appointments.
"We have continued to improve the IT system that operates for the vaccination programme," the statement read.
"As GP and pharmacies operate on different IT systems there may be a gap in notification of the vaccination which can, from time to time, result in multiple appointments.
"Once the vaccination is put on the system, any previous appointments issued from the vaccination centres will be cancelled by the HSE."
Booster planning inadequate - Shortall
The co-leader of the Social Democrats has said that there has been inadequate planning for the Covid-19 vaccine booster programme by the HSE.
Róisín Shortall told RTÉ's News at One that it was not a case of "no shows" clogging up the system, but that there were problems in recording the numbers who had been vaccinated by GPs and pharmacists.
She said that the HSE have to ensure they can cope with the demand if they are to run walk-in booster clinics.
Ms Shortall said that this morning's clinic at UCD was only available for two hours for the over 50s and had insufficient staff available to run it.
She said the HSE should bring in the army and other potential staff to roll out the clinics given the high demand, or else to reconsider having walk-in clinics and operate on appointment-only basis.
Ms Shortall said that more than 500 pharmacists are providing booster jabs, yet there was no information given to the public about this, adding that there is "real miscommunication around this".