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

With the recent spread of the Omicron variant that has lower symptom severity rates than conventional strains, and the assumption that a vast majority of patients who had recently being enrolled in the trial having infected by the Omicron strain, Fujifilm decided that, even if trials were continued under the current clinical trial protocol, it would be difficult to verify Avigan’s efficacy to suppress the symptoms from becoming severe, and that continuation of the placebo control trial would not lead to the subjects’ benefits. FUJIFILM Toyama Chemical has therefore decided to terminate the enrollment of new subjects into this trial. The clinical data of the patients enrolled in the trial will be analyzed, going forward, in compliance with the clinical trial protocol, the company said in a statement.

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.

Former Prime Minister Shinzo Abe said he hoped Avigan would be approved in May 2020 for COVID-19, and he pledged to 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.