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Pfizer Inc has launched early-stage human trials of an experimental oral drug that could be prescribed to patients at the first sign of infection with COVID-19.

The drugmaker, which developed the first authorized COVID-19 vaccine in the U.S. with Germany's BioNTech SE, said the antiviral candidate showed 'potent' activity against the virus in lab studies.

Pfizer's candidate, which is called PF-07321332, belongs to a class of drugs known as protease inhibitors.

The pill would work by inhibiting an enzyme that the coronavirus uses to make copies of itself inside human cells.

 Protease inhibitors have been effective at treating other viral pathogens such as HIV and hepatitis C virus, both on their own and in combination with other antivirals, the company said.

Pfizer believes this class of molecules may provide well-tolerated treatments against COVID-19, as currently marketed therapeutics that work on the same lines have not reported safety concerns.

'Tackling the COVID-19 pandemic requires both prevention via vaccine and targeted treatment for those who contract the virus,' said Pfizer's chief scientific officer, Mikael Dolsten, in a statement. 

'Given the way that SARS-CoV-2 is mutating and the continued global impact of COVID-19, it appears likely that it will be critical to have access to therapeutic options both now and beyond the pandemic.' 

The Phase I trial is a randomized, double-blind trial in which some participants will be given the drug and others the placebo, but even researchers won't know what pill the volunteers.

If results show the medication is safe and effective, the company will move on to Phase II and recruit a larger group pf participants. 

It's currently unclear how long the treatment course would be and how many times per day or per week patients would take the pill. 

The company is also studying an intravenously administered antiviral candidate in an early-stage trial in hospitalized COVID-19 patients.

'Together, the two (oral and intravenous candidates) have the potential to create an end-to-end treatment paradigm that complements vaccination in cases where disease still occurs,' Dolsten said.