Professor Luke O'Neill has said that mask-wearing on Irish public transport should stay "for definite".
NPHET are due to meet today to discuss lifting the mandate on mask-wearing in retail settings, schools and on public transport.
The Trinity College immunologist said that because public transport is an enclosed space, it's the "perfect place for the virus to spread".
He said on Newstalk today: "I'd mandate them on public transport for definite.
"We know from other studies years ago: toy mainly catch colds and flus on public transport in winter cause you're crammed in together.
"One reason why flu went down, we think, is less people were on public transport and we were wearing masks. So I'd certainly maintain mask-wearing on public transport.
"Other settings it'll have to become optional - I mean we can't have laws about everything, can we?"
Professor O'Neill said it "makes sense" to delay lifting mask-wearing on public transport until winter has passed.
"My other concern is we're still in the middle of winter, remember.
"I think they should maybe leave it a few weeks really until the winter's passed.
"The virus is still rampant out there - it makes sense to me to leave it a few more weeks."
However, speaking in the Dail today Minister Eamon Ryan said he hopes the requirements for mask-wearing in schools, public settings and transport can be removed "straight away",
Minister Ryan said during Leaders Questions: "My view is that we should start immediately to remove requirements in school. I have the same view for retail and public transport."
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