Almost 1 million people haven't come forward for their first Covid booster, according to new HSE figures.
Just under 4 million people over-12 got their primary vaccination - but less than 3 million have got their third dose.
It comes as 606 Covid patients are in hospital - over three times as many as at the start of the month.
Trinity College neuro-scientist, Tomás Ryan, says the situation is worsening.
"Based on hospitalisation numbers it's clear we're in a new wave of Covid infection in Ireland.
"The most important thing that we can do is increase vaccination, that means a fourth dose for the elderly and the vulnerable, and it also means vaccinating under 12s who haven't been vaccinated yet."