Professor Luke O'Neill says a new pill 'is the one they're looking for' to fight COVID-19.
MSD - or Merck - says that molnupiravir reduced the risk of hospitalisation or death by approximately 50%.
While its trial of the drug was stopped, due to the positive nature of the results.
Merck says it plans to submit an application for emergency use to the US Food and Drug Administration (FDA) "as soon as possible" - as well as marketing applications "to other regulatory bodies worldwide".
Prof O'Neill told Pat Kenny this is all looking very positive.
"This is the one they're looking for - antiviral.
"They've been looking for antivirals to kill the virus anyway, as well as vaccines.
"Remdesivir was the first one that came along - this beats remdesivir into a cocked hat, basically.
"50% decrease in hospitalisation, which is an amazing thing, so it just shows you this could be the one.
"But they got so excited that they stopped the trial half-way through, because it would have been unethical to continue because it was working in the trial."
He says the drug prevents you from replicating the virus.
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"They planned to recruit another 750 people, didn't need to because the statistics and the effect was so dramatic.
"And it was remarkable numbers: four times a day you take a tablet, they gave it to people who were high risk...over-60s.
"And they halved the rate of hospitilisation and no deaths at all in the treated group.
"And the mechanism is clear - it stops replication, so the virus can't divide.
"If you take these tables, the virus can't divide anymore".
And he says this has two consequences for people who take the pill.
"One is you won't get sick, obviously, but secondly you won't spread it because the virus is being killed in your body so it stops transmission as well."
Asked if the virus could find a way around this mechanism, Prof O'Neill says it is unlikely.
"We treat HIV/AIDS with an antiviral, hepatitis C we use [an antiviral] - we never discovered a vaccine for AIDS still.
"They're just managed with antivirals, and there's no evidence of escape from those.
"It's such a profound, dramatic drug you can't escape it is the idea here.
"In other words we shouldn't see resistance building up, which is great".
And he says the price tag on the new drug should be low.
"It should be cheap, these are very cheap to make these tables... and there'll be pressure on Merck to make it really cheap.
"And by the way, the Biden administration had given US$3.2bn towards discovering this kind of drug - some of that went to Merck. So it'll be very, very cheap."
And he says the next move is up to the FDA.
"Merck are asking for emergency use immediately, they've gone to the FDA right now and given them all their data... and if they get emergency use, it'll be launched very quickly.
"Merck are already scaling up production; they're making 10 million doses as we speak to begin getting the drug out.
"Again we wait for the FDA, because they have the final decision but it's heading in the right direction".