Pfizer is to temporarily reduce the production of its covid-19 vaccine.
The company says this will affect shipments later this month.
Meanwhile, the gap between the two doses is being increased from 21 to 28 days in the Republic of Ireland.
The decision has been made so more people can get the initial protection from the virus.
The first doses of the Moderna vaccine will be administered tomorrow.
They will be used to vaccinate GP teams at mass vaccination centres in three locations.
The clinics will be in Dublin, Galway and Portlaoise.
Taoiseach Micheal Martin says the country remains on course to have 4 million people vaccinated against Covid-19 by September.
He told an IIEA webinar "we do have the capacity to achieve that" and says the approval of the AztraZenica vaccine later this month will be a "significant game-changer".