It is not realistic to have widespread vaccination against coronavirus by April, according to Tomás Ryan.
The associate professor in the School of Biochemistry and Immunology at Trinity College Dublin told Newstalk Breakfast this will take longer.
"COVID is on it's way out, but it could be a longer time than we think.
"We're talking about coming out of this lockdown in early April or late April: we will have nowhere near widespread vaccination by that point in time.
"If we come out of this lockdown and we don't have low enough case numbers, we can easily be heading quickly towards a fourth wave if - with this new variant - there's too much of it in the population".
Host Ciara Kelly said Ireland is projected to have 1.25 million vaccine doses given by the end of March, and over two million by the end of April.
On this, Prof Ryan said: "We will not have half the population vaccinated or infected or anything near it by April.
"Even with the most ambitious targets by Micheál Martin - which are based on optimistic supply - we might have half the population having a single dose of the vaccine by the middle of June".
"Having half the population vaccinated by April, I don't think that's realistic".
'Risky situation'
But he suggested if this was the case, it could present "a very risky situation" where large numbers of the population are vaccinated and others are not.
"Internationally, this is going to create the ideal situation for variants to emerge that are vaccine resistant."
But he said some aspects are improving.
"Things are definitely improving, probably not [at] as fast a rate as some people would like.
"We're definitely seeing a lot of very heterogonous improvement in the sense that the 14-day average is looking really good in places like Cork, Kerry, Kilkenny and other counties.
"Whereas on the other side of the spectrum places like Dublin, Galway and Offaly cases are not really declining so much.
"This lockdown is a longer lockdown than any of us would have liked - that's almost certainly due to the B117 variant... which is now dominant in the population - which means it is going to be a slightly longer lockdown, or the alternative is we could find ways of going harder".
On Tuesday, Taoiseach Micheál Martin said over 80% of adults will have received the first dose of a COVID-19 vaccination by the end of June.
He said 1.25 million doses will have been administered by the end of March - then one million doses on average per month between April and June.
That will mean up to 40% of people over 18 will have received their first dose by the end of April, up to 64% by the end of May, and up to 82% by end of June.
Health officials have also said the vaccine list is "constantly being reviewed", after people with 'very high risk' conditions were moved up in priority.
It means people under aged 16 to 69 with very high-risk conditions will now be offered the vaccine in the fourth phase, after the over-70s.
