Many people in their 60s won't be eligible for a COVID-19 booster vaccine "just yet", the HSE has said.
The extra dose can only be given at least five months after the second dose, so it could be early next year before many people get their booster.
As of this week, boosters have been approved for everyone over 50, healthcare workers, and anyone over 16 with an underlying health condition.
Around 500,000 boosters or additional doses have been given so far.
However, HSE CEO Paul Reid told Newstalk Breakfast people may need to wait a bit longer for an extra vaccine dose, even if they're in the age groups now getting boosters.
He explained that there's at least a five-month gap needed between the second vaccine dose and a third booster vaccine.
Most people in the 60s will have received the AstraZeneca vaccine, which had a longer gap between doses than the Pfizer and Moderna jabs.
That means for many in their 60s the five-month period may not have passed yet.
Mr Reid said: “There will be a lag effect into March of next year, where that five-month period kicks in. And many healthcare workers would have joined us later this year, so their eligibility period will carry into early next year.
“We will be making contact with you [if you're eligible]. We’re in a much stronger position now - we have people’s contact details. We will be contacting those people as we work through their age cohorts.
“Bear with us - we will be working through them all."
He said there is concern around high levels of non-attendance at some booster vaccine clinics.
He urged people to come forward when they are offered a booster, and called on businesses to make sure staff can take the time to attend their booster appointments.
This week, NIAC and the Chief Medical Officer said people who were given the Janssen COVID-19 vaccine can get a booster after just three months.
Many teenagers and people in their 20s will have received that vaccine, as it was available to those age groups before the mRNA vaccines.
Mr Reid said they're still formally waiting to receive the advice from NIAC and the CMO around the criteria for giving boosters to those younger people who receive the Janssen vaccine.
However, he said there is "emerging and strong" evidence from across Europe of waning immunity from the one-shot jab.