Drugmakers Pfizer and Merck have begun late-stage trials to test antiviral drugs against SARS-CoV-2, the virus that causes COVID-19, the companies reported Wednesday.

The drugs, each given orally as a pill, would provide an unprecedented COVID-19 preventive medication option if brought to market.

Merck’s drug, molnupiravir, developed in collaboration with Ridgeback Biotherapeutics, will be given to study participants aged 18 and older who reside in the same household as someone with an early, laboratory-confirmed SARS-CoV-2 infection with at least one sign or symptom. The approximately 1,332 participants will be randomized to receive either 800 mg of molnupiravir or placebo orally every 12 hours for five days. 

Investigators will track how many participants become infected with the virus within 14 days of receiving the drug and experience adverse events after taking the drug, Merck stated.

Pfizer’s drug PF-07321332, meanwhile, will be tested in non-hospitalized, symptomatic adult participants who already have a confirmed diagnosis of SARSCoV-2 infection and are not at increased risk of progressing to severe illness, the company reported. The trial will enroll approximately 1,140 participants. These patients will receive the antiviral pill plus the drug ritonavir (to enhance the effect of the investigational drug) or placebo orally every 12 hours for five days. 

If successful, PF-07321332/ritonavir has the potential to provide patients with an oral therapy that could be prescribed at the first sign of infection, without requiring hospitalization, Pfizer said.

Both drugs use technology that blocks the ability of a virus to replicate and spread further throughout the body.