Pfizer Inc has started an early-stage U.S. trial of an oral COVID-19 antiviral therapy that could be prescribed to patients at the first sign of infection, the company said on Tuesday.

The drugmaker, which developed the first authorized COVID-19 vaccine in the U.S. with Germany’s BioNTech SA, said the antiviral candidate showed potent activity against SARS-CoV-2, the virus that causes COVID-19, in lab studies.

Pfizer’s candidate, named PF-07321332, is a protease inhibitor that prevents the virus from replicating in 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’s candidate is behind two other oral antiviral therapies, which are in mid-stage trials – the first being developed by rival Merck & Co with Ridgeback Bio, and a second from Roche Holding and Atea Pharmaceuticals.

Gilead Sciences’ remdesivir is currently the only U.S. Food and Drug Administration-approved drug for the treatment of COVID-19.

The FDA has granted an emergency authorization to intravenous therapies from Eli Lilly – bamlanivimab alone and in combination with etesevimab, and a combination therapy from Regeneron.