About 30 percent of the influenza viruses that have spread across the country this winter seem to be resistant to treatment with Tamiflu, the drug most commonly prescribed for flu, the Health, Labor and Welfare Ministry said Friday.

Because Tamiflu is so widely used in medical institutions across the country, the ministry has alerted them to the situation and advised additional care be taken in choosing which drugs to use to treat flu. The virus confirmed to be resistant to Tamiflu is influenza A virus subtype H1N1, which accounts for an estimated 36 percent of three main strains of flu spreading in the country today.

According to the National Institute of Infectious Diseases, of viruses that were found in 35 flu patients reported to the institute through 11 prefectural governments by Jan. 8, those found in 34 people were confirmed to be the drug-resistant virus, accounting for 97 percent.

The rate is much higher than the 2.6 percent recorded last winter. The ministry intends to study the conditions of the drug-resistant patients and the cures by forming a research group as well as pursuing laboratory testing in the remaining 36 prefectures.

According to the ministry, another drug, Relenza, is effective against the Tamiflu-resistant virus and no drug-resistant virus has been found in flu strains other than H1N1, including influenza A virus subtype H3N2 and influenza B virus.

The drug-resistant virus was reported all over the world last winter, and it has already been found in the United States and Britain this winter. The virus also was reported in countries where Tamiflu is not widely used. It seems the virus became resistant to the drug through natural mutation in a certain gene.