COVID vaccine maker Pfizer (PFE) has announced that it started Phase 2/3 trials of an oral antiviral drug to prevent COVID-19 infections.

The trial is part of a global study of up to 2,660 patients that are 18 and older and live in the same household with individuals who have a confirmed case of the virus and are experiencing symptoms.

The oral drug, PF-07321332, is being administered to healthy trial participants in a low dose twice daily for five or 10 days to prevent COVID infections and their symptoms through day 14.

The drug is being combined with ritonavir, an older medication that was used in combination for the treatment of HIV infections, Reuters reported.

In the Phase 1 clinical trial the oral drug proved safe and well-tolerated, Pfizer said.

“If successful, we believe this therapy could help stop the virus early – before it has had a chance to replicate extensively – potentially preventing symptomatic disease in those who have been exposed and inhibiting the onset of infection in others,” Mikael Dolsten, chief scientific officer and president of Worldwide Research, Development and Medical at Pfizer, said in a statement.

Pfizer is not the only drugmaker that is testing an oral drug to prevent COVID-19 as Merck and Roche are also working to develop an antiviral pill for coronavirus prevention.

Merck, which has partnered with Ridgeback Biotherapeutics, announced earlier in September that it has begun late-stage trials of its oral drug molnupiravir for the prevention of COVID-19. Roche said in June that its AT-527 oral drug produced positive interim results from a global Phase 2 study in hospitalized patients with mild-to-moderate COVID-19.

“If successful, molnupiravir could provide an important additional option towards reducing the burden of COVID-19 on our communities.” Dr. Nick Kartsonis, senior vice president, vaccines and infectious diseases, clinical research, Merck Research Laboratories, said in a statement at the time.

As of Monday at 10:26 a.m., shares of Pfizer were trading at $44.03, down 9 cents or 0.19%.

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