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CDC reports multiple cases of oseltamivir-resistant influenza A H1N1
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In mid-September 2009, the CDC confirmed two additional cases of oseltamivir-resistant influenza A H1N1.
The two patients were both adolescent girls who had contracted the virus at a summer camp in North Carolina. Both girls had received oseltamivir during a mass chemoprophylaxis program in which approximately 600 campers and staff members received a neuraminidase inhibitor.
In July, the North Carolina State Laboratory of Public Health confirmed pandemic influenza A H1N1 virus infection in respiratory specimens from both girls. A few weeks later, CDC researchers detected the H275Y mutation (N1 numbering) in neuraminidase from both specimens. The H275Y mutation is associated with resistance to oseltamivir. CDC researchers said a second mutation, I223V, in neuraminidase also was detected in both specimens.
This is the first report of oseltamivir-resistance in pandemic influenza A H1N1 cases with an epidemiologic link, according to information from the CDC.
CDC officials are now warning health care workers to be aware that antiviral resistance can develop during chemoprophylaxis or treatment with subtherapeutic dosages and that they should follow published recommendations for antiviral medications.
Resistance to oseltamivir
According to information from the CDC, initial testing of the 2009 pandemic influenza A H1N1 virus indicated that it was susceptible to neuraminidase inhibitors, including oseltamivir and zanamivir, and was resistant to adamantanes, such as amantadine and rimantadine.
Neuraminidase inhibitors have been used widely for treatment and chemoprophylaxis of 2009 pandemic influenza A H1N1. But a few cases of oseltamivir-resistant 2009 pandemic influenza A H1N1 virus infection have been reported worldwide, including several in the United States. Oseltamivir-resistant influenza A H1N1 was also confirmed in two immunosuppressed patients in Seattle a few days before the confirmation of the North Carolina cases. The cases are not thought to be related.
CDC officials said the recent reports of cases of oseltamivir-resistant influenza A H1N1 indicate that oseltamivir chemoprophylaxis may not be the optimal treatment for all patients with influenza A H1N1 and warned that resistance to oseltamivir might develop during subtherapeutic dosing.
CDC recommendations
The CDC’s recommendations regarding use of antivirals in patients with influenza A H1N1 were updated on Sept. 8. “Use of antiviral medications for postexposure chemoprophylaxis should be reserved for persons at higher risk for influenza-related complications who have had contact with someone likely to have been infected with influenza,” the recommendations state. “An emphasis on early treatment once a patient has developed symptoms, rather than chemoprophylaxis, should reduce opportunities for development of oseltamivir resistance. Chemoprophylaxis should not be used for prevention of illness among healthy persons after exposures in community settings.”
For more information:
- MMWR. 2009;58:969-972.
- MMWR. 2009;58:893-896.
- MMWR. 2009;58:913-918.
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