There is "very strong evidence" backing the need for people over the age of 65 to come forward for their second booster.
Registration for the second booster yesterday opened for the over 65s and people with weak immune systems.
The first appointments are going ahead at vaccination centres around the country today - with GPs and pharmacies due to join the rollout in the coming weeks.
The Head of the HSE vaccine programme Dr Damien McCallion said there is ‘very strong’ evidence backing the benefits of the second booster.
"I can’t emphasise that enough," he said. "They did a very thorough international review.
"They looked at the evidence and they were very strong in relation to asking people to come forward.
"I can’t stress strongly enough that the medical evidence and the advice from our key medical staff is very clear that in order to prevent hospitalisation, severe illness and death that people do come forward."
He said there are around 750,000 people eligible for the second booster, including 50,000 immunocompromised people.
He said the booster is only available to people who got their last booster more than four months ago and has not been COVID-positive within the last four months.
Mr McCallion said it is still not clear whether people will need to be boosted a third time when winter sets in - but the HSE is prepared for that eventuality.
"What NIAC (National Immunisation Advisory Committee) has said is, they haven’t concluded on that at the moment," he said.
"There are some observations in the recent advice in relation to the potential for an autumn vaccination programme and we have already been working for the last couple of months on a possible plan for that.
"We are looking at how we can align that with the flu from a practical perspective. So, we are just currently finalising plans even in the absence of clear advice so we are prepared for whatever may come in the autumn."