The HSE says work on protecting its IT system from a major cyber attack is making slow progress.
Yesterday's ransomware attack paralysed national and local networks, and led to the cancellation of some services.
Appointment delays are expected, with the executive stating it will take days to restore its systems.
The vaccination programme is unaffected, however anyone with Covid-19 symptoms is asked to attend a walk-in centre, as the GP referral system for testing is down.
Chief Operations Officer for the HSE, Anne O'Connor is pleading those with existing vaccination and Covid test appointments to proceed as normal.