Almost 9,000 people received a first dose of a Covid-19 vaccine at walk-in centres over the weekend.
HSE Chief Executive Paul Reid says two thirds of those who showed up hadn't previously registered for a vaccine.
The first vaccines for those aged 12 to 15 are expected to be administered this weekend, with the portal opening on Thursday.
Immunologist Professor Luke O'Neill says attention needs to turn to booster shots next.
"If you're an immunologist you'd always say use boosters, because they always really get things going.
"It's a bit like a refresher course for the immune system.
"The two shots you give get the whole thing going, and everything is firing and it's fine. But then it wanes a bit, or it becomes less strong over time.
"Then you give a booster, and everything is really high again."
