The Chief Medical Officer says it's possible people will need more than one booster of a Covid-19 vaccine.
The booster campaign is underway for healthcare workers, people over the age of 50 and the medically vulnerable.
NPHET is to meet again on Thursday to see if it needs to make further recommendations to Government.
Dr Tony Holohan says it's hoped immunity from boosters is longer-lasting than the initial vaccine.
"We have hope, but we'll need the longitudinal studies to tell us that the benefit of a boosted population will last longer than the initial vaccination, and that's what our hope will be.
"There is a possibility that we will need further boosters down the line. We can't say exactly what the duration will be. It's not an unlikely scenario."
Meanwhile booster vaccinations are available to the over 60s on a walk in basis at Cork City Hall today.
People over 60 can attend without an appointment up until 4pm, provided that it has been at least five months since their last vaccination dose.
Anyone over 12 that needs a first or second dose of a Covid vaccine can also attend today.
