Regulators here are investigating a possible link between an Irish patient's blood clot and the AstraZeneca vaccine.
It's the first "case of interest" that's been sent to the Health Protection Regulatory Authority (HPRA) here.
The European Medicines Agency said yesterday there was a possible link between the vaccine and cerebral venous sinus thrombosis (CVST).
Over 200,000 doses of the formula have been given here - around a fifth of the vaccines administered so far.
In a statement, the HPRA said: "The HPRA can confirm that it has received a case of interest, associated with the AZ vaccine, which describes CVST, an unusual clot in the brain.
"We are following up on this report to obtain additional details and to evaluate if it is consistent with the profile of rare blood clotting events that were the focus of the recently concluded EMA safety review.
"This report will be notified to the EU's safety database and will be considered in the context of continuous monitoring coordinated by the EMA."
Dr Lorraine Nolan, the CEO of the HPRA, says there is no new guidance yet about the use of the AstraZeneca vaccine.
She stressed scientists believe a link between the jab and very rare clotting events is plausible, but it's not yet fully proven.
Yesterday, the EMA said it had found a 'possible link' between the vaccine and very rare cases of unusual blood clots with low blood platelets.
It said clotting should be listed as a rare side effect of the vaccine.
However, it added that the overall benefits of the vaccine outweigh the risk of side effects.