Newly released studies have eased fears about COVID-19 breakthrough infections, Professor Luke O'Neill says.
A breakthrough infection refers to a COVID-19 case in someone who has been fully vaccinated.
While all the current vaccines are extremely effective, none have 100% efficacy - meaning there'll always be some people who will still test positive.
The rapid spread of the more transmissible Delta variant has also led to concerns that more vaccinated people will get the virus than previously anticipated.
However, Professor O'Neill - Professor of Biochemistry at Trinity College - told The Pat Kenny Show that new research has brought some good news.
He explained: “They’ve looked very closely at people who have been vaccinated and then get infected. We know a certain number will pick up an infection, because of Delta which is so transmissible.
“[Researchers] followed really closely healthcare workers who had been vaccinated and then who had got reinfected. They measured lots of things, and they saw two very important things."
One key finding was that people had symptoms for less time than if they were unvaccinated - and those symptoms were typically minor.
Perhaps even more importantly, there was less 'virus shedding' - in other words, vaccinated people were spreading COVID-19 less.
Professor O'Neill said: “It’s evidence the vaccine is really working, even if it isn’t 100%. If it does break through, there’s a residual effect of the vaccine which protects.
“In other words, our fear of breakthrough infections has lessened because of that study - even though it does happen, there’s still protection going on.”
A number of recent studies have looked specifically at the issue of breakthrough infections.
One study published in The Lancet Infectious Disease journal last week found that "almost all symptoms were reported less frequently in infected vaccinated individuals than in infected unvaccinated individuals".
It also found that vaccinated people were more likely to be "completely asymptomatic".
Professor O'Neill also addressed questions around booster vaccines and vaccinating younger age groups.
In terms of boosters, the leading immunologist said a third shot is a "really powerful way" to get the immune system going.
While full research on the efficacy of booster shots has not yet emerged, Professor O'Neill said health officials are "erring on the side of caution" and now looking at giving vulnerable groups an additional shot.
He said: “Even if those people, after the third shot, pick up an infection, there’s a very low chance it’ll progress to severe disease”.
Meanwhile, he said the main reason many countries - including in Ireland - have chosen to vaccinate younger age groups is “to try to get rid of the virus more and more from our communities”.
He said COVID-19 is usually a very mild illness in teenagers, but health authorities believe vaccinating over-12s “will help us really eliminate this virus”.
He also noted: “You [also then] won’t need to vaccinate the under-12s - the more you vaccinate over-12s, the [less] spread there will be to younger groups.”