The company's oral antiviral treatment has entered a mid-to-late-stage trial and is being tested on adults with COVID-19 symptoms who have not been treated in hospital and are not at risk of becoming severely ill.
Pfizer is one of several drug manufacturers hoping to develop the first antiviral pill to be taken for early signs of the illness.
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The study will involve 1,140 participants and will combine the treatment, PF-07321332, with a low dose of ritonavir, which has previously been used with other antiviral medications.
Patients will receive the treatment or a placebo orally every 12 hours for five days.
PF-07321332 is designed to block the activity of the main enzyme that is needed for the virus to replicate.
If the trial is successful, Pfizer said it would file for emergency approval between October and December this year.
The drug manufacturer said it could be prescribed at the first sign of infection, without requiring hospitalisation.
The company is running a separate trial involving people who are not in hospital but are at risk of becoming severely ill, which began in July.
There are no antiviral treatments for coronavirus that have been approved for use in the UK.
Gilead Sciences' remdesivir, administered intravenously, is the only approved antiviral treatment for COVID in the US.
Another COVID-19 pill, molnupiravir, by Merck and its partner Ridgeback Biotherapeutics, is already being studied in a late-stage trial in patients who are not in hospital to see if it lowers the risk of hospitalisation or death.
A separate late-stage trial is looking at whether taking molnupiravir can prevent people from being infected with COVID-19.
The prime minister, Boris Johnson, said in April that he hoped pills or capsules could be available by the autumn to help fight a possible further wave of the virus.
A government taskforce was launched to identify the most promising antiviral treatments - particularly those that can be taken at home - and support their development through clinical trials.