Professor Luke O'Neill says data from Israel shows how much higher the risk of serious illness from COVID-19 is among unvaccinated people.
Israel led the world in the initial vaccine rollout, vaccinating a large percentage of its population in the first few months of the year.
Like many other countries, Israel is now battling a fresh surge in cases attributed to the Delta variant.
The number of new cases rapidly increased during August, with a five-day average of over 7,000 cases now being reported.
This led to some initial fears that the vaccine may not have been effective as thought, or that protection may wane over time.
Professor O'Neill told The Pat Kenny Show the data coming from Israel has been very informative.
He explained: “It turns out, only slightly over half the population was fully vaccinated - there wasn’t quite as high a level of vaccination as other places.
"They opened up very quickly as well… that meant spread could happen, especially with Delta.
“The serious cases are nine times higher in the unvaccinated [over-60s] than in vaccinated - in other words, the vaccines are working to protect against serious disease. Then in the under-60s, it’s double the rate in the unvaccinated population.
“In other words: get the vaccine. The evidence is very clear there."
The Trinity College immunologist says it's believed Delta arrived in the country when it was brought in by people who went on holiday.
For now, the data has persuaded the Israeli government to become one of the first in the world to start a booster campaign of third vaccine doses for older and vulnerable people.
Do vaccines wane?
Meanwhile, research has been continuing internationally about how long vaccine protection lasts.
One study published last week, for example, indicated that the protection offered by the Pfizer vaccine decreases at a faster rate than AstraZeneca.
Professor O'Neill said this is the key question in the field of immunology at the moment.
He said: "If vaccines wane, when do we boost and who do we boost?
“Endless studies keep coming out… a very interesting study from a US group has shown you get really good responses out to six months if you’re a healthy person. We’re not talking about vulnerable people or immune-deficient people.
“What’s very interesting… the antibodies may go down a bit, but the cells that make them are still there. They become what are called memory b-cells, and they’re waiting for you to get infected - then they’ll make loads more antibodies.
“As the months go by, the b-cells get stronger - which is a very interesting conclusion. They’re making antibodies that are even better as time goes by."
He said evidence so far has shown there is 'a little waning' when it comes to vaccine efficacy over time.
However, it also points to a durable response among healthy people - and the vaccines are also working to protect against the more transmissible variants of the virus seen so far.