Influenza A viruses (H1N1 subtype) that are resistant to the drug oseltamivir circulated widely in the U.S. during the 2007-2008 influenza season, with an even higher prevalence of drug resistance during the current 2008-2009 influenza season, according to a study to be published in the March 11 issue of JAMA, the Journal of the American Medical Association.
During the 2007-2008 influenza season, increased levels of resistance to the influenza drug oseltamivir (marketed as Tamiflu) were detected for the first time in the United States and worldwide. In addition, early 2008-2009 influenza season surveillance data suggest that oseltamivir resistance among influenza A(H1N1) viruses will most likely be higher, according to background information in the article. It was unknown whether some resistant viruses would cause clinical illness similar to other influenza viruses.
Nila J. Dharan, M.D., of the Centers for Disease Control and Prevention, Atlanta, and colleagues examined the trends and characteristics of patients infected with oseltamivir-resistant and -susceptible influenza A(H1N1) virus. These viruses, identified and submitted to the CDC by U.S. public health laboratories between September 2007 and May 2008 and between September 28, 2008, and February 19, 2009, were tested as part of ongoing surveillance.
During the 2007-2008 season, influenza A(H1N1) accounted for an estimated 19 percent of circulating influenza viruses in the United States. Resistance to oseltamivir was identified among 142 of 1,155 U.S. influenza A(H1N1) viruses (12 percent) tested during the 2007-2008 influenza season. Data were available for 99 persons infected with oseltamivir-resistant influenza and 182 persons infected with oseltamivir-susceptible influenza from this period. Among resistant cases, median (midpoint) age was 19 years, 5 patients (5 percent) were hospitalized, and 4 patients (4 percent) died. No significant differences were found between cases of oseltamivir-resistant and oseltamivir-susceptible influenza in demographic characteristics, underlying medical illness, or clinical symptoms. The researchers did not find an association between use of oseltamivir and cases of illness due to infection with oseltamivir-resistant A(H1N1) viruses in the United States.
Preliminary data from the early 2008-2009 influenza season indicates that oseltamivir resistance among A(H1N1) viruses continues at high levels. As of February 19, 2009, resistance to oseltamivir had been identified among 264 of 268 (98.5 percent) U.S. influenza A(H1N1) viruses tested.
"The emergence of oseltamivir resistance has highlighted the need for the development of new antiviral drugs and rapid diagnostic tests that determine viral subtype or resistance, as well as improved representativeness and timeliness of national influenza surveillance for antiviral resistance," the authors write.