Professor Luke O'Neill says Sweden now appears to be 'doing better' in terms of the number of new coronavirus cases than they were before.
However, he said the main question for scientists now is why that is the case.
Sweden has faced intense scrutiny - including from Professor O'Neill - during the pandemic for its comparatively light restrictions, with officials controversially indicating they were aiming for a 'herd immunity' approach.
The high death rates in the country compared to its Scandinavian neighbours saw heavy international criticism of the Swedish approach.
The country has reported over 5,800 deaths since the pandemic began - compared to only 622 in Denmark, 344 in Finland and 264 in Norway.
However, authorities in Sweden have now claimed their relatively low current case rate is evidence their approach has worked.
Speaking on The Pat Kenny Show, Professor O'Neill - professor of biochemistry at Trinity College Dublin - said scientists are looking at Sweden very closely.
He explained: "Sweden is an absolute fascination for us scientists - we're looking at it very closely.
"You can only compare it to other Scandinavian countries... there are less variables. There's so many complexities with this virus that there's no point in comparing Sweden with Brazil.
"Comparing it with Denmark... there was a much higher death rate. Sweden was criticised for this, and rightly so. They said they were playing the long game."
He suggested the Swedish authorities 'let too many people die' with their light-touch approach to restrictions and lockdown.
However, at the moment evidence is pointing to them having fewer cases - which should translate to fewer deaths.
Situation in Sweden
Professor O'Neill said there are a few possible explanations for the current situation in Sweden, where there is now a lower new case rate than a number of other European countries.
He explained: "The view of immunologists would be there's a chance... that they've built up a little immunity, and therefore you're seeing less cases.
"The second [possibility] is they're just behaving themselves: Sweden is a very different place to Ireland culturally, so the Swedes are inclined to be more compliant with what they're being told to do. That could be another reason why the case rate is down.
"Certainly they're doing better now in terms of number of cases - the big question is why."
Professor O'Neill said there were big outbreaks in nursing homes and migrant communities in Sweden, and the reason for the high death rate was because specific populations were being affected.
He said: "It's a long game here, and the ultimate death toll will be some number in the future.
"It depends now not so much on case numbers... ultimately, if they have a lower death rate overall, they may have built up a bit of immunity to protect people."
However, he said if a therapy emerges to effectively treat COVID-19 cases in the near future then the high death rate in Sweden will have been "needless".
