BEIJING, April 4 (Xinhuanet) -- A Japanese study published in the April 4 issue of the Journal of American Medical Association found that virus variants that cause influenza B have become resistant to two common influenza drugs named Relenza and Tamiflu.

    Both antiviral drugs Tamiflu, manufactured by Roche and Relenza, manufactured by GlaxoSmithKline, are neuraminidase inhibitors, which have been effective against influenza and are used extensively. Tamiflu-resistant viruses that cause influenza A have been detected and reported early. But rare evidence has been reported to suggest that influenza B viruses have been mutated to resist these drugs.

    According to the researchers, the pathogenicity or virulence of the mutated viruses is not weakened by the mutation that caused the resistance to Tamiflu and or Relenza, in contrary to the common perception of mutated strains.

    "Influenza B mutants with reduced sensitivity to neuraminidase inhibitors are circulating, and these viruses can cause infections with no difference in duration of symptoms, level of viral shedding, or clinical outcome," Anne Moscona, M.D., of Weill Medical College of Cornell University, New York, and Jennifer McKimm-Breschkin, Ph.D., of Molecular and Health Technologies, Parkville, South Victoria, Australia write in an accompanying editorial.

    "In light of the recent observation that oseltamivir (Tamiflu) may be less effective against influenza B than against influenza A, an important concern is whether suboptimal dosing for these viruses will lead to increased selection of viruses with high-level resistance."

    Tamiflu is indicated for treating adults, adolescents, and pediatric patients one year of age and older with the influenza whose symptoms started within the last day or two. Tamiflu is also used to reduce the chance of getting the influenza in people aged one year and older who have a higher chance of getting the influenza because they spend time with someone who has the influenza.

    Tamiflu can also reduce the chance of getting the influenza if there is an influenza outbreak in the community. The use of Tamiflu to reduce the chance of getting influenza has been studied up to 42 days in adults and up to 10 days in children.

    Relenza is indicated for use to treat uncomplicated illness due to influenza virus in people seven years and older who have been symptomatic for no more than two days. Relenza is also used preventively, to reduce the chance of getting influenza illness, in people five years and older.

    Shuji Hatakeyama, M.D., Ph.D., of the University of Tokyo, Japan, and colleagues examined the prevalence and transmissibility of influenza B viruses with reduced sensitivity to neuraminidase inhibitors in Japan where both Tamiflu and Relenza are used more extensively than anywhere else in the world.

    "Continued surveillance for the emergence or spread of neuraminidase inhibitor¨Cresistant influenza viruses is critically important," Hatakeyama and colleagues write. "Further evaluation of the biological properties of neuraminidase inhibitor¨Cresistant influenza viruses is needed to fully assess their pathogenicity in humans."

    Experts urge reduced use of Tamiflu to retain its antiviral power over influenza viruses as it is widely recognized as the sole weapon that can be used in the case of next pandemic bird flu, which is expected to have the potential of killing millions of people worldwide. Tamiflu can shorten the duration and lessen symptoms of influenza, but can't prevent influenza.

    (Agencies)