Pfizer said that its trial will enroll 1,140 non-hospitalized adults diagnosed with the SARS-CoV-2 infection who are not at risk of severe illness. The patients in the trial will be given the experimental pill and a low dose of ritonavir, an older medication used in combination treatments for HIV infection. The drug, known as PF-07321332, is designed to block the activity of a key enzyme that helps the virus multiply.

The company began a different trial of PF-07321332 in July, in adults with COVID-19 infection who are at high risk of becoming severely ill due to underlying conditions. The company expects initial results from the study in the coming fall. If the trial is successful, Pfizer said it would file for a potential emergency use authorization by the fourth quarter.

Merck, meanwhile, said that its new trial will study the drug molnupiravir to prevent COVID-19 among adults in the same household as someone diagnosed with symptomatic infection. The drug is an antiviral designed to introduce errors into the RNA of the virus, eventually preventing it from replicating. Merck and its partner, Ridgeback Biotherapeutics are already conducting a late-stage trial of the treatment in non-hospitalized patients.

Merck also said in June that the government agreed to pay about $1.2 billion for 1.7 million courses of molnupiravir, if proven to work, and if authorized by regulators. The company is expected to file for U.S. emergency use authorization in the second half of 2021 at the earliest. Early trial results for molnupiravir yielded promising results, and experts believe it could help cut back on the time that infected individuals remain positive for the virus.