Travel was a "key factor" in the rise of the second wave of the coronavirus pandemic across Europe, according to Professor Luke O'Neill.
The Immunology expert from Trinity College Dublin said that Ireland is one of the few countries in the EU which did not have a worse second wave than its first.
New data shows that 105,000 people have died as a result of the second wave in Europe.
Professor O'Neill said that 60% of cases across the bloc came from people going on holidays in Spain during the summer.
Speaking on The Pat Kenny Show, he said we should be proud of how Ireland managed to control the spread of COVID-19.
He said: "Most countries in Europe have had a second wave worse than their first, apart from Ireland.
"Ireland is singled out with Norway and Finland as doing really well in the second wave, our cases did not go up hugely like they did in other countries.
"So we get a gold star for our performance which is tremendous for our people and the way we behaved."
Holidays
Professor O'Neill said the numbers regarding the second wave "are stark" since early June when Europe emerged from the first wave and the EU changed its policy.
After initially warning that the EU should be restrictive in allowing widespread movement of people during the summer, EU Commission President Ursula von der Leyen changed her stance said we can use "smart solutions" in order to go on holidays.
Professor O'Neill explained: "At the start of June people started to travel again, the borders open and then we see a spread begins to emerge.
"It looks like the travel part of this seems to be a key factor.
"This is especially evident with Spain, with four million people travelling there and then returning home while there was no test and trace system in place.
"Now we know 60% of cases in Europe came from the Spanish vacation and spread throughout Europe.
"The consequence of this was the second wave was caused effectively in part by these summer vacations."
He said that "we should be quite proud of ourselves in the sense that we managed to get it somewhat under control compared to these other countries".
"The science of the second wave shows us that travel was a big factor, nobody can disagree with that, and then very poor test-trace-isolate protocols when people went back to their home countries," he added.
Christmas
Professor O'Neill said that similar to Spain being a popular holiday destination during the summer, now the big concern is the ski resorts during the winter.
His second big fear is Christmas and people mixing and mingling.
It comes as health officials will today discuss a sudden rise in coronavirus patients admitted to intensive care yesterday.
Eight people were admitted to ICU with the virus yesterday – the highest number in 24-hours since the middle of Spring.
Professor O'Neill added: "It's a really worrying few weeks ahead of us, I think in Ireland we have to optimistic and hope people do adhere to the guidelines because they will certainly help."
