Over 2 million certs have been issued to date, with 1.1 million sent out by email.
Extra agents are working on the government helpline today to deal with the large number of calls from people who want to travel.
The HSE's Damien McCallion says they're trying to clear the backlog:
"We still have a small proportion, particular going back to the very early days of the programme, when there was pressure on hospitals, long term care, health care workers and so on and where minimal information was provided or incorrect data. Probably what we've left now was in around about 150,000 of those that we're working through. We have a team and that's reducing every day."
