It's going to very difficult to achieve herd immunity, according to immunology professor Paul Moynagh.
It comes as the European Medicines Agency (EMA) is meeting to discuss extending the use of the Spikevax - previously called Moderna - vaccine to include children aged 12 to 18.
This would add to Pfizer/BioNTech, which has already been approved for use in those aged 12 to 15.
The efficacy of Pfizer was calculated in close to 2,000 children who had no sign of previous infection.
Of the 1,005 children receiving the vaccine, and not a placebo, none of them developed COVID-19 compared to 16 children out of the 978 who received a dummy injection.
This means that the vaccine was 100% effective at preventing COVID-19 - however the true rate outside the study could be between 75% and 100%.
But Prof Moynagh told Newstalk Breakfast the benefit of the vaccine is less to younger age groups.
"The EMA's already approved Pfizer for the use in children from the age of 12 to 15, that's currently under consideration by NIAC.
"So when you look at the use of vaccines in various age groups, it's important to look at the benefit to risk ratio.
"So far when you've applied that to all age groups in the adult cohorts, the benefits outweigh the risks.
"With children, that benefit to risk ratio gets smaller - with younger age groups... children are reasonably resistant to the virus, and especially in terms of developing severe COVID, although it can happen in some cases.
"The other slight concern is that, in some cases, it's been observed that with the RNA vaccines - including the Pfizer vaccine - there is a condition known as myocarditis, which is inflammation of cardiac or heart muscle.
"That's an incidence of about one in 20,000, so that has created some concern... so that narrows that gap in terms of the benefit to risk".
And he says any drive towards herd immunity would have to include children.
"If we're looking at situation where we can to try to reach herd immunity, certainly you'd have to vaccinate children.
"Now is herd immunity achievable, and I think that's the question now".
'We wouldn't be able to get to 90%'
He says such herd immunity would require "around 90%" of the population to be vaccinated.
"We're talking about approving a vaccine for 12 to 15-years-olds, but again we wouldn't be able to get to 90%.
"The other consideration is the fact that some of these variants can... cause infection to those that have been vaccinated.
"I think we could be looking at a situation where it's going to be very difficult to achieve herd immunity.
"I think, globally over the coming months and years, we're probably going to be looking at a situation where everybody will end up either being vaccinated, or actually infected by the virus or in some cases both".
And he says the winter months will likely see the unvaccinated most susceptible to the virus, which will be children.
"Even though the virus is transmitting really well, I think in the winter months it's going to transmit even better.
"So even though only a small number of children will end up being affected by severe COVID, a small percentage of a really large number is still a significant number.
"So I think it needs to be really closely considered, I'd probably like to see more data".
On Thursday, HSE chief clinical officer Dr Colm Henry said children will likely have to be vaccinated for Ireland to reach herd immunity.
He said the threshold for reaching herd immunity is higher because of the new strain.
"It was 60-70% of the population... but because we're dealing with a more transmissible variant, the estimate has gone up to 85-90%.
"If the estimate is that high... it would infer we would need to include age groups going down right to children. That's based on the current estimate of herd immunity that might be needed to deal with the increased transmissibility of Delta", he added.
