By Omar Abdel-Baqui and Joseph Walker 

An oral Covid-19 treatment developed by Atea Pharmaceuticals Inc. failed in a midstage trial, the company said Tuesday, sending the company's stock tumbling.

Atea, in collaboration with Swiss pharmaceutical company Roche Holding AG, said its treatment didn't cut the viral load in nonhospitalized patients with mild to moderate symptoms.

Boston-based Atea's stock plunged 73% in Tuesday morning trading.

Despite the negative news, the company said its data suggested that its AT-527 treatment has antiviral activity in high-risk patients with underlying health conditions.

The results come after Merck & Co. and its partner Ridgeback Biotherapeutics LP said earlier this month that their experimental Covid-19 pill cut the risk of hospitalization or death by about 50% in high-risk people with mild to moderate Covid-19. They have since filed an application asking U.S. health regulators to authorize their antiviral drug, molnupiravir.

Atea and Roche said they are considering modifying their Phase 3, or final-stage, study of the drug based on the results from the Phase 2 study. Modifications could include changes to which patients are enrolled and the study's primary goal. Results are now expected in the second half of 2022; analysts had previously been expecting data this year.

Antiviral drugs often are most effective early in the course of disease and show the greatest benefit in patients whose own immune systems aren't adequately fighting the virus. It can be difficult to show significant improvements in drug trials across all patients because many will get better on their own.

Write to Omar Abdel-Baqui at omar.abdel-baqui@wsj.com and Joseph Walker at joseph.walker@wsj.com