The period of time between people receiving the two doses of a Covid-19 vaccine is being increased to 28 days from the current 21 days.
The Health Service Executive said the decision to increase the dose interval was taken following updated advice from the World Health Organization and the National Immunisation Advisory Committee (NIAC) here.
In a statement, the HSE said "information about the vaccine and the programme is evolving, the WHO and NIAC recently provided updated advice, stating that the interval between the two doses has been increased from 21 days to 21-28 days."
As a result, the HSE said it has decided to implement the change for everyone receiving their first vaccine dose from next Monday, 18 January onwards.
The HSE said the move "will enable us to maximise our resources and vaccinate more people as new supplies of the vaccine come into the country".
Anyone who has already received their first dose of the Covid vaccine will still receive their second dose as originally planned after 21 days.
The HSE said "this new change will not have an impact on the protection it offers".
All residents of long-term care facilities, who have already received patient information, will be sent new information leaflets to allow them to inform residents and staff of this change from 18 January.
The HSE also said updated aftercare leaflets will be available to issue to those who receive the vaccine from Monday.
Meanwhile, the head of Europe's medicines regulator has said she is hopeful it will give approval to the AstraZeneca vaccine by the end of the month.
The biopharmaceutical company, which developed the vaccination with Oxford University in England, formally applied to the European Medicines Agency earlier this week.
Executive Director of the EMA Emer Cooke said the agency started the official review on Tuesday and she said it has "put a tentative timetable in for the end of January".
The EMA is due to meet on 29 January to make a decision on the conditional market authorisation application.
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Speaking following a virtual address to the Institute of International and European Affairs in Dublin this morning, Ms Cooke said the EMA is receiving new information all the time and the agency hopes to have sufficient data to be able to come to a scientific conclusion by the end of the month.
She said "it is a challenge but I am hopeful."
But Ms Cooke warned that "things can go wrong" and she said "the dates depend on everything going right."
Asked about when the EMA expects Johnson & Johnson to seek regulatory approval for its vaccine, Ms Cooke said she hopes it will happen next month.
She said while she expects them to seek approval when they are ready, "we hope it is to come in February, but whether I could confirm that that's the case, I'm afraid I'm not in a position to do so".
Ms Cooke also said the companies that have already received conditional market authorisation will not have to go through the entire clinical trials process again to show their vaccines are effective against the new variants of Covid-19.
She said "we are working with Pfizer/BioNTech and Moderna precisely on how their vaccines would work in the context of the new strains".
She said the EMA hopes it will be able to do "a very adapted process".
Ms Cooke said the companies "wouldn't have to go back to the full package" and she said "we will be able to use a lot of what we have already".
1,300 vaccines to be administered to GPs
More than 1,000 vaccine doses are to be given to general practitioners tomorrow, a Donegal-based GP has said.
Speaking on RTÉ's Drivetime, Dr Denis McCauley said there will be three vaccination centres in operation this weekend.
The 1,300 vaccines due to be given tomorrow will be given to GPs and their practice nurses, and GPs who are attaching themselves to nursing homes, he said.
He said the rest of the GP teams will be vaccinated through the hospital-based system or mass vaccination system.
Dr McCauley there will not be "an order" to how GPs are being vaccinated.
"It's very much a case of we are all healthcare professionals and we are all equal."
He said he expects "most of the GPs in Donegal will be vaccinated by the end of next week", and he believes this will be "replicated around the country."