A professor of immunology says the Omicron variant will become the dominant strain in Ireland within the next 10 days.
Professor Christine Loscher was speaking as the wait time between a second COVID-19 vaccine and a booster has been reduced from five to three months.
Details were announced by Health Minister Stephen Donnelly on Monday following new NIAC advice.
"The gap between completion of primary schedule of COVID-19 vaccination and a booster dose will be reduced to three months", he said.
"Boosters will continue to be offered in the priority order previously recommended."
Up until now, people vaccinated with anything but Janssen had to wait a minimum of five months after being deemed fully vaccinated before getting their booster.
It comes as the Chief Medical Officer Dr Tony Holohan says the Omicron variant now accounts for 11% of COVID-19 cases here - up from less than 1% a week ago.
Prof Loscher is a professor of immunology at Dublin City University (DCU).
She told Newstalk Breakfast the next two weeks will be crucial for the booster campaign.
"If there's nothing else that we do in the next 10 days before Christmas, everything has to go in to maximising the number of boosters that we get into arms in the next two weeks.
"Omicron has accelerated much more quickly than we thought, and we do know that that third dose is vital to controlling case numbers.
"And I think we'll see that that 11% is going to climb very quickly in the next week or two.
"We will definitely have dominance in the next 10 days.
"We don't know enough to not be concerned - but I think there is good optimistic data that it may be milder, and that may be a good thing for us".
She says shortening the booster waiting period makes sense.
"I think it's really significant - it comes off the back of the head of vaccine strategy of the EMA... recommending that the booster interval be lessened.
"We're responding to the acceleration of Omicron, where we know that booster dose is absolutely vital.
"And the second thing is that new science has come to hand where this... study in the UK has shown categorically that after two weeks, you get as good a response with the booster as you would a few months later.
"So I suppose its allowed us to be able to really lessen that interval, without having a huge impact on losing any of the benefits of boosting".
J&J 'should be prioritised'
Prof Loscher believes it is right to boost older people - and those with underlying issues - first, with one exception.
"Coming down through those cohorts in the way that we did before is sensible; but the people who got J&J [Janssen] - we know that their immunity has waned much faster and is much much lower.
"So I think they should be prioritised, regardless of age, and that is the one cohort I think we should focus on".
She says this one-shot vaccine was one of the first offered in pharmacies.
"It was one of the first ones made available through the pharmacies, and a lot of young people took it - it was a single shot.
"But I suppose the science has moved on now and we know now that that has waned the quickest in terms of immunity.
"So I really think that they need to skip that age queue".