Professor Sam McConkey has said Ireland's slower roll-out of coronavirus vaccines is not the fault of Health Minister Stephen Donnelly.
Professor McConkey is head of the department of International Health and Tropical Medicine at the Royal College of Surgeons in Ireland (RSCI).
He told The Hard Shoulder: "The limiting factor on this... is actually supply chain.
"There isn't enough, we can't just get as much Pfizer vaccine as we'd like today, so we're limited by the supply - as is everyone else in the world.
"Supply chain is a big problem with these vaccines".
"I would have a lot of sympathy for people who are involved in this, because the complexity of manufacturing enough is difficult, and predicting will the yield from a vaccine manufacture process be 1,000 doses or 500 doses is difficult in advance.
"It's like brewing wine - you don't know quite next year what your yield will be - or even on a dairy farm, how much milk will you be able to produce in six months time? You don't quite know".
"And then of course every country's looking for it: there's a little bit of what I would call vaccine nationalism, where people say 'no we want it before you'.
"We haven't always found the most civilized ways of negotiating fair ways of supply".
"So this is a world-wide supply chain problem that doesn't have an easy fix, so I don't believe Stephen Donnelly is guilty of this supply chain problem.
"It's just a a difficult thing to make and get foresight on".
He said the biggest priority over the next few weeks is to vaccinate those aged over-85s.
"Over-85s will be proactively contacted by their GP practice to say 'come in and have the vaccine'.
"And indeed if there are some extra vials over it maybe then that some over-80s or maybe over-70s can be on standby to come in and get those doses".
It comes as people over 85 were due to start getting their first vaccine doses from Monday, in the first step of the community rollout of vaccines.
Around 13,500 people in this age cohort will receive the Moderna or Pfizer vaccine this week.
While the vaccination programme continues in care homes - where remaining residents and staff will soon be getting their second doses - and among frontline healthcare workers.
And the Government has revealed where mass COVID-19 vaccinations centres will be located across the country.
Thirty-seven centres will be set up, with at least one in every county. Five sites have been identified in Cork, along with four in Dublin.
Counties Kerry, Tipperary, Westmeath and Wicklow will also have two vaccination centres each.