US pharmaceutical company Pfizer Inc (NYSE:PFE) has started an early-stage US trial of an oral COVID-19 antiviral therapy that could be prescribed to patients at the first sign of infection, Reuters news agency reported on Wednesday.

Pfizer said on Tuesday that its antiviral candidate, PF-07321332, a protease inhibitor that prevents the virus from replicating in cells, showed potent activity against SARS-CoV-2, the virus that causes COVID-19, in lab studies.

According to Pfizer, 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.

Pfizer added that it 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.

In addition, the company is also studying an intravenously administered antiviral candidate in an early-stage trial in hospitalised COVID-19 patients.

Pfizer's Chief Medical Officer stated: "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."