A group of researchers says tests of two coronavirus drugs on hamsters indicate that the risk of drug-resistant variants emerging is limited.

The study was conducted by Professor Kawaoka Yoshihiro from the University of Tokyo's Institute of Medical Science and other researchers.

They used hamsters with suppressed immune systems and infected them with the Omicron variant of the coronavirus.

The group then administered Lagevrio, which is also known as molnupiravir, and another drug developed by Japanese pharmaceutical firm Shionogi. Lagevrio is already in use. An application for Shionogi to be used in Japan has been submitted.

The results for both drugs showed that the amounts of the virus in the hamsters' lungs dropped to one 10,000th on the second day of treatment. But some amounts of the virus remained because of the suppressed immune systems.

On Day 9, the researchers checked the therapeutic efficacy of the drugs against the virus extracted from the hamsters. They say they found no resistant variants.

The group says some antiviral drugs can cause resistant variants to emerge in several days.

Kawaoka says further studies are needed to see if results will be the same in humans, but at least the tests on animals suggest that the two drugs carry a low risk of leading to resistant variants.