Professor Luke O'Neill says there's "no question" NIAC will approve the use of the AstraZeneca and Johnson & Johnson vaccines for younger people.
The Trinity immunologist says it would be a "scandal" to leave unused vaccines sitting on the shelf.
Currently, it's recommended that the two vaccines only be used for those aged 50 or over.
With the vaccine rollout soon to extend to younger age groups, the Government and health officials are expecting updated advice from NIAC in the coming days.
Professor O'Neill told The Pat Kenny Show he's confident they'll give the green light for the wider use of the two jabs.
He said: “They’re definitely going to approve it for younger people - they have to. There’s no question.
“They’ll have to decrease the age to use up the supply. Otherwise there’s a risk of leaving vaccines on the shelf, which would be a scandal.
"Give it to the developing world if you’re not going to use it yourself."
Currently, those who get a vaccine appointment do not have a choice over which vaccine they receive.
Professor O'Neill said that may change if Ireland soon has lots of supply available.
He noted that trials are currently underway to see if different vaccines can be “mixed and matched”, but current data supports the single vaccine protocol being used at the moment.
One of the concerns around the rollout is the different dose timelines for the various vaccines.
Johnson & Johnson is a single jab, so people are considered fully vaccinated shortly after receiving it.
For Pfizer and Moderna, people will have to wait around a month for their second dose.
However, that wait is three or four months for those who receive the AstraZeneca vaccine.
The 'vaccine bonuses' - such as being able to visit the homes of other vaccinated people - apply to fully vaccinated people, or 28 days after the first dose of AstraZeneca.
Professor O'Neill said he'd press for the bonuses to apply for all vaccines a few weeks after a single shot, and he believes the EMA should push for that approach.
He said: “That’d be good - especially for travel.
"With the summer coming, if you have to wait 16 weeks… then the summer’s gone.”
It comes as the Tánaiste yesterday said everyone who wants a vaccine may be offered one before the end of June.
Health Minister Stephen Donnelly today said that goal is "very ambitious" - but it's one he hopes they can deliver on.
Flu and COVID-19 combination jab
With four vaccines currently approved for use in the EU, the vaccine produced by Novavax is expected to be one of the next to be approved in the coming months.
In the meantime, the firm has started work on a jab that would combine the flu and COVID-19 vaccines in a single shot.
Professor O'Neill explained: "They’ve got great pre-clinical data - they’re testing it in animals initially.
“They got a tremendous result to the flu and COVID-19 in the one shot.
"What they’re saying the future could well be flu and COVID combinations.”
The jab likely won’t be used this winter, but it could be used in the coming years - particularly if people need booster shots against COVID-19
Professor O'Neill said the Novavax flu jab is “already one of the best flu vaccines out there”.
He noted that their COVID-19 vaccine technology is based on the COVID-19 spike protein itself, unlike the other vaccines currently in use.