Photo/IllutrationXofluza (Asahi Shimbun file photo)

Viruses that are resistant to Xofluza, widely used to treat influenza, turn out to be just as infectious as non-resistant viruses and are often more virulent, researchers say.

A study by the University of Tokyo in conjunction with other institutions disproved the traditional notion that resistant viruses are less infectious.

The research team's findings were published Nov. 25 in the science journal Nature Microbiology.

To prevent the possible spread of resistant viruses, experts are urging doctors and parents to exercise caution when using Xofluza for children as they are prone to develop resistant viruses.

The researchers led by Yoshihiro Kawaoka, a professor of virology at the University of Tokyo's Institute of Medical Science, conducted the study last winter.

They separated resistant viruses from four patients who had taken Xofluza from non-resistant viruses that were present before the treatment.

The researchers infected hamsters, mice and ferrets with the viruses to ascertain whether the two types of viruses have different levels of intensity of infection and symptoms.

The results showed that resistant viruses triggered weight loss in the infected rodents in almost exactly the same manner as non-resistant viruses. The resistant viruses multiplied in the lungs and throats of the tested animals just as non-resistant viruses do. The researchers also confirmed that ferrets became infected with resistant viruses when placed in a cage next to an infected ferret.

Xofluza hit the market in March 2018. According to the health ministry, the drug had a nearly 40-percent share in the anti-influenza drug market last season because a single tablet is sufficient to treat flu symptoms.

A clinical study showed that resistant viruses are highly likely to occur in users of the drug. Such viruses were observed in 23.4 percent of those under the age of 12, whose immune systems are not fully developed.

Guidelines published by the Japan Pediatric Society in October caution against administering Xofluza to those under the age of 12.

The guidelines also call for careful monitoring of children to check for the possible emergence and transmission of resistant viruses.