Flu strains resistant to antiviral drugs such as Tamiflu and Rapiacta were detected in six people in Sapporo in November and December, according to the National Institute of Infectious Diseases.
The strains share roughly identical gene sequences and are believed to be spreading in the city, the institute said Monday. Relenza and Inavir, antiviral medicines with different action mechanisms, are effective in fighting the detected varieties, it said.
"In cases when (viruses) are suspected to be resistant to drugs, it's necessary to consider using different types of antiviral drugs," an official with the institute said.
The six infected patients were four children aged 10 or younger, and two adults, according to the national institute and the Sapporo City Institute of Public Health.
The strains all belong to the H1N1 type and are marked by genetic mutations indicating that they are resistant to Tamiflu and Rapiacta. Tests confirmed that the potency of these two drugs was limited to 1/500th on these strains compared with others.