Some Drug Resistance To Influenza B Medications

Two antiviral drugs, zanamivir and oseltamivir, which are a type of drugs known as neuraminidase inhibitors, have been effective against influenza and are used extensively. There has been documented evidence of the emergence of oseltamivir-resistant type A viruses, but similar information on influenza B viruses has been limited. Influenza B viruses are associated with annual outbreaks of illness and increased death rates worldwide, according to background information in the article.

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 zanamivir and oseltamivir are now used more extensively than anywhere else in the world. In the winter of 2004-2005, an influenza B virus caused a widespread epidemic in Japan, creating an opportunity to assess the effectiveness of neuraminidase inhibitors.

The researchers collected influenza B isolates from 74 children before and after oseltamivir therapy and from 348 untreated patients with influenza (including 66 adults). Four hundred twenty-two viruses from untreated patients and 74 samples from patients after oseltamivir therapy were analyzed.

The researchers identified a variant with reduced drug sensitivity in one (1.4 percent) of the 74 children who had received oseltamivir, and seven (1.7 percent) of the 422 influenza B viruses isolated from untreated patients were found to have reduced sensitivity to zanamivir, oseltamivir, or both. Review of the clinical and viral genetic information available on these seven patients indicated that four were likely infected in a community setting, while the remaining three were probably infected through contact with siblings shedding the mutant viruses.

"Continued surveillance for the emergence or spread of neuraminidase inhibitor--resistant influenza viruses is critically important," the authors write. "Further evaluation of the biological properties of neuraminidase inhibitor--resistant influenza viruses is needed to fully assess their pathogenicity in humans."

JAMA. 2007;297:1435-1442.