A simple pill to treat the coronavirus at home has been sought since the start of the global health crisis. So far all treatments have been either intravenous or vaccine shots.
Pfizer's is the second anti-COVID pill after that of Merck, which is actually an influenza medicine rebranded to fight the coronavirus. Pfizer's has been created specifically to fight COVID.
The Pfizer drug called Paxlovid achieved an 89 percent reduction in risk of hospitalization or death among adult patients with COVID who are at high risk of progressing to severe illness, the US company said.
The results from the middle-to-late stage clinical trial were so strong that Pfizer will stop recruiting new people for the trial, it said.
Pfizer will submit the data to the Food and Drug Administration as soon as possible as part of its "rolling submission" for Emergency Use Authorization.
"Today's news is a real game-changer in the global efforts to halt the devastation of this pandemic," said Pfizer CEO Albert Bourla.
"These data suggest that our oral antiviral candidate, if approved or authorized by regulatory authorities, has the potential to save patients' lives, reduce the severity of COVID-19 infections, and eliminate up to nine out of 10 hospitalizations," he added.
The main analysis of the data looked at numbers from 1,219 adults in North and South America, Europe, Africa, and Asia.
In the days immediately after symptoms appeared, some of them were given the experimental drug and others got a placebo – for five days, every 12 hours.
"The scheduled interim analysis showed an 89 percent reduction in risk of COVID-19-related hospitalization or death from any cause compared to placebo in patients treated within three days of symptom onset," Pfizer said.
Ten people who took the placebo died, while among those who got the Pfizer medication, none did.
Pfizer had planned a testing pool of 3,000 people but stopped when it got to 70 percent of that because the results of the medication were so promising.
The search for a pill
Several companies are working on so-called oral antivirals, which would mimic what the drug Tamiflu does for influenza and prevent the disease from progressing to severe.
Britain on Thursday became the first country to approve an anti-COVID pill, as it greenlit the use of Merck's antiviral drug called molnupiravir to treat patients suffering from mild to moderate coronavirus.
Pfizer's product is known as a "protease inhibitor" and has been shown in lab testing to jam up the virus' replication machinery.
If it works in real life, it will likely only be effective at the early stages of infection.
By the time COVID progresses to severe disease, the virus has largely stopped replicating and patients suffer from an overactive immune response.
Until now, COVID therapeutics such as monoclonal antibodies and Gilead's remdesivir – authorized for use in the EU under the name Veklury – have been administered intravenously.
Merck's molnupiravir was initially developed as an inhibitor of influenza and respiratory syncytial virus – two other important acute respiratory infections – by a team at Emory University in Atlanta, Georgia.
Britain, which has been one of the countries hardest hit by the pandemic, announced on October 20 that it had ordered 480,000 doses of molnupiravir.
Pfizer is carrying out two other clinical trials with its pill: among people who are not at risk of progressing to severe COVID, and among people close to someone with COVID to see if it protects them against the illness.
Besides Pfizer and Merck, the Swiss pharma giant Roche is also working on a COVID pill.