A drug once hailed as Japan's biggest contribution in the global fight against COVID-19 suffered another setback on Friday, as Fujifilm Holdings Corp. said it was halting enrollment in a clinical trial of the pill, Avigan.

The emergence of the omicron variant of coronavirus, which usually causes milder infections, made it tough to determine the drug's effectiveness in preventing severe symptoms, the company said in a statement.

Clinical data from patients already enrolled in the trial will continue to be analyzed, it said.

In the first few months of the pandemic, Japan ordered Fujifilm to triple national stockpiles of the drug, which had originally been approved as an emergency flu treatment.

Then-Prime Minister Shinzo Abe said he hoped Avigan would be approved in May 2020 for COVID-19, and he pledged donate supplies to needy countries.

Known generically as favipiravir, the drug was eventually approved in India and Russia, based on early studies showing it eased the symptoms of COVID-19 and cut hospital stays.

But studies in Japan have been inconclusive, leading domestic regulators to hold off approval for it, partly because animal study data showed it could lead to birth defects.

As Avigan stumbled in clinical trials, other antiviral pills have come to the fore against COVID-19. Japan has bought millions of doses of oral treatments developed by Pfizer Inc. and Merck & Co.

A third option, developed by domestic firm Shionogi & Co., was submitted for regulatory approval last month.