Mutated influenza viruses resistant to baloxavir marboxil, a new flu drug sold as Xofluza, were detected in patients last month, the National Institute of Infectious Diseases has said.

This was the first confirmation by a national research institute of such mutations in flu viruses since the drug was put into practical use last March, the health ministry said.

According to the NIID, Xofluza-resistant viruses were found in two of four primary school students in Yokohama in generic screenings in December conducted after they developed flu symptoms earlier the same month.

The mutated viruses were 76 to 120 times more resistant to the new anti-flu drug developed by Shionogi & Co. than unmutated ones detected in the other two children.

In clinical trials of baloxavir marboxil, resistant viruses were detected in 23.4 percent of participating patients younger than 12 years old.

The NIID said that it will continue to keep an eye on mutated flu viruses and provide related information swiftly.

Sales of Xofluza have been increasing because only one dose of the drug is enough for treatment, according to the drug maker and other sources.

The health ministry said last week that the number of influenza patients per medical institution in Japan in the week through Jan. 20 had hit the second-highest level since the survey started in 1999.

The average number of flu patients at some 5,000 medical institutions across the country that are regularly monitored grew by 15.37 from the previous week to 53.91, following the record high of 54.33, marked in the previous year, the ministry announced Friday.

The total number of flu patients during the week is estimated at some 2.13 million, up by some 495,000 from the previous week.

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