Oseltamivir-resistant novel influenza A (H1N1) virus infection has been identified in the United States for the first time, in two severely immunosuppressed patients in Seattle.
The two cases—a teenaged male and a woman in her 40s—are both leukemia patients who have undergone hematopoietic stem cell transplants. Both were initially infected with oseltamivir-susceptible viruses which later developed resistance during antiviral treatment, the Centers for Disease Control and Prevention reported.
The two patients were treated at different hospitals and were not linked epidemiologically. There was no evidence of transmission of the oseltamivir-resistant virus to health care providers or contacts of either patient, the CDC said.
In both patients, the viruses were found to be susceptible to zanamivir by neuraminidase inhibition assay. Sequence analysis showed that the oseltamivir resistance was not the result of gene reassortment with seasonal influenza A (H1N1) virus.
The CDC recommends that immunosuppressed patients receive annual influenza vaccination, even though in some the immune response can be decreased. Clinicians caring for immunosuppressed patients who are infected with novel H1N1 should be aware of the potential of antiviral drug resistance during therapy and prolonged viral shedding, the CDC said.
The public health risk of virus transmission from these two cases appears to be low. Washington state, in collaboration with CDC, is conducting enhanced surveillance for oseltamivir resistance among novel H1N1 virus strains isolated from patients in hospitals and in the community.
Oseltamivir or zanamivir are recommended for treatment of all hospitalized patients with suspected or confirmed novel H1N1 and for outpatients at increased risk for influenza-related complications.
Zanamivir should be considered the treatment of choice in immunosuppressed patients with oseltamivir-resistant novel H1N1 infection, except for those with underlying airway disease.
Guidance on treatment and prevention is available atwww.cdc.gov/h1n1flu