There will be 30,000 fewer people vaccinated against Covid-19 than planned this week due to the pause on using the AstraZeneca vaccine.
The decision was taken to halt its use while an investigation is carried out to determiner whether a small cluster of serious blood clot events in Norway were completely random or linked to the vaccine.
Approximately one in five of the total vaccine doses given in Ireland so far have been the AstraZeneca formula, however, it was due to account for a much bigger share this week as the vaccination of medically vulnerable people was expected to be ramped up.
The HSE was planning to give 80,000 doses this week, but has now reduced this week's estimates to 50,000.
The Minister for Health, Stephen Donnelly says blood clots have been observed before, but nothing more than expected.
"What was different about the notification from Norway was there was blood clotting in the brain, which obviously can be serious, and that it was in younger people.
"[The National Immunisation Advisory Committee] and the [Health Products Regulatory Authority] met, and they considered that, and they have taken a very cautious approach," Mr Donnelly said.
In a statement last night, AstraZeneca said there had only been a small number of blood clotting cases among the 17 million people who have received the vaccine in the EU and UK to date.
The drugmaker said there has been 15 cases of deep vein thrombosis and 22 of pulmonary embolism in that group up to last week.
"This is much lower than would be expected to occur naturally," the company said.
Immunologist, Professor Kingston Mills, said people should not jump to conclusions.
"Sometimes it is very difficult to link event that happen after vaccination to the actual vaccine, because those things can happen in the general population, and this could be one of them.
The vaccination of elderly people with the Pfizer and Moderna jabs will go ahead as scheduled.