A trial of Pfizer's experimental antiviral pill for Covid-19 was stopped early after the drug was shown to cut by 89% the chances of hospitalisation or death for adults at risk of developing severe disease, the company said.
The results appear to surpass those seen with Merck's pill molnupiravir, which was shown last month to halve the likelihood of dying or being hospitalised for Covid-19 patients also at high risk of serious illness.
Full trial data is not yet available from either company.
Professor Luke O'Neill said the new drug is "very significant" because the manufacturer got 89% efficacy in their trial.
The Trinity immunologist added: "What that means is 9 out of 10 people wouldn't end up in hospital with Covid. If that turns out to be true once it's launched that would be remarkable.
He said this disease is an emergency and Pfizer is applying for emergency use in the United States with the FDA and also with the European Medicines Agency (EMA).
"Those agencies will look very closely at the data and the safety and they will be really under pressure now approve these drugs because an antiviral is a great extra weapon to use against this virus," Prof O'Neill said.
Separately, the Tánaiste has said a new antiviral oral pill for Covid will be a "very valuable weapon" and he hopes it can be approved by the EMA "quite soon".
Yesterday, the UK medicines regulator became the first to approve the drug 'Molnupiravir', for people who have had a positive Covid test and have at least one risk factor for developing severe illness, such as obesity, being over the age of 60, diabetes or heart disease, something Leo Varadkar said was "really encouraging".
Mr Varadkar said: "Once you are diagnosed you can take this tablet and it reduces by up to half the chances of you needing to be hospitalised, so that can really make a big difference.
"I hope the EMA will approve that quite soon, because you can never deal with a virus through vaccination alone, you need therapeutics too and you need preventative medicine.
"This is going to give us an extra weapon in our armoury and a very valuable weapon too," he added.
Pfizer will now submit interim trial results for its pill, which is given in combination with an older antiviral called ritonavir, to the US Food and Drug Administration as part of the emergency use application it opened in October.
The combination treatment, which will have the brand name Paxlovid, consists of three pills given twice daily.
A spokesperson from Pfizer in Ireland said that the company's plant in Ringaskiddy "will support the global manufacturing and supply" of the drug, if it is given approval.
He said: "Pfizer has begun investing prior to regulatory authorisation … in the manufacture of our potential Covid-19 oral antiviral candidate to help bring this potential treatment to patients as soon as possible.
"Pfizer's site in Ringaskiddy has a successful history of contributing to our manufacturing efforts."
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The planned analysis of 1,219 patients in Pfizer's study looked at hospitalisations or deaths among people diagnosed with mild to moderate Covid-19 with at least one risk factor for developing severe disease, such as obesity or older age.
It found that 0.8% of those given Pfizer's drug within three days of symptom onset were hospitalised and none had died by 28 days after treatment.
That compared with a hospitalisation rate of 7% for placebo patients. There were also seven deaths in the placebo group.
Rates were similar for patients treated within five days of symptoms - 1% of the treatment group was hospitalised, compared with 6.7% for the placebo group, which included ten deaths.
Antivirals need to be given as early as possible, before an infection takes hold, in order to be most effective. Merck tested its drug within five days of symptom onset.
"We saw that we did have high efficacy, even if it was five days after a patient has been treated ... people might wait a couple of days before getting a test or something, and this means that we have time to treat people and really provide a benefit from a public health perspective," said Annaliesa Anderson, head of the Pfizer programme.
The company did not detail side effects of the treatment, but said adverse events happened in about 20% of both treatment and placebo patients.
"These data suggest that our oral antiviral candidate, if approved by regulatory authorities, has the potential to save patients' lives, reduce the severity of Covid-19 infections, and eliminate up to nine out of ten hospitalisations," said Pfizer Chief Executive Albert Bourla.
Infectious disease experts stress that preventing Covid-19 through wide use of vaccines remains the best way to control the pandemic, but only 58% of Americans are fully vaccinated and access in many parts of the world is limited.
Pfizer's drug, part of a class known as protease inhibitors, is designed to block an enzyme the coronavirus needs in order to multiply.
Merck's molnupiravir has a different mechanism of action designed to introduce errors into the genetic code of the virus.
Merck has already sold millions of courses of the treatment, which was approved this week by UK regulators, to the US, the UK and others.
Britain said earlier this month it had secured 250,000 courses of Pfizer's antiviral.
Pfizer is also studying whether its pill could be used by people without risk factors for serious Covid-19 as well as to prevent coronavirus infection in people exposed to the virus.