The Health Service Executive considering whether to recall hundreds of patients whose flu vaccines were incorrectly administered.
It is thought that some adults who got the vaccine from pharmacists could have been given doses designed for children.
The mistake occurred after pharmacists received inaccurate information during training.
The HSE is considering giving those affected a second dose of the vaccine, which they say poses no health risk.
Training body, Hibernian Healthcare, has apologised for any upset caused to patients.