During October 2010–July 2011, 1.0% of pandemic (H1N1) 2009 viruses in the United States were oseltamivir resistant, compared with 0.5% during the 2009–10 influenza season. Of resistant viruses from 2010–11 and 2009–10, 26% and 89%, respectively, were from persons exposed to oseltamivir before specimen collection. Findings suggest limited community transmission of oseltamivir-resistant virus.
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No association was found between this increase in oseltamivir resistance and prior oseltamivir use (11,13). Oseltamivir resistance in pandemic (H1N1) 2009 and seasonal influenza A (H1N1) viruses was conferred by the H275Y substitution in the neuraminidase. However, unlike seasonal influenza A (H1N1) viruses, which retained susceptibility to the M2-blocking adamantanes (amantadine and rimantadine), >99% of circulating pandemic (H1N1) 2009 viruses are inherently resistant to adamantanes (14). Thus, inhaled zanamivir or investigational drugs, such as intravenous zanamivir, are the only treatment options for patients with oseltamivir-resistant pandemic (H1N1) 2009 virus infection.
Our conclusions are limited by the small number of patients with oseltamivir-resistant pandemic (H1N1) 2009 virus infection. Variability in state surveillance and the number of specimens tested from each state may also have limited the representativeness of our data.
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Aaron D. Storms, Larisa V. Gubareva, Su Su, John T. Wheeling, Margaret Okomo-Adhiambo, Chao-Yang Pan, Erik Reisdorf, Kirsten St. George, Robert Myers, Jason T. Wotton, Sara Robinson, Brandon Leader, Martha Thompson, Marjorie Shannon, Alexander Klimov, and Alicia M. Fry, for the US Antiviral Resistance Surveillance Working Group1
Author affiliations: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (A.D. Storms, L.V. Gubareva, S. Su, J.T. Wheeling, M. Okomo-Adhiambo, A. Klimov, A.M. Fry); California Department of Public Health, Sacramento, California, USA (C.-Y. Pan); Wisconsin State Laboratory of Hygiene, Madison, Wisconsin, USA (E. Reisdorf); New York State Department of Health, Albany, New York, USA (K. St. George); Maryland Department of Health and Mental Hygiene, Baltimore, Maryland, USA (R. Myers); Minnesota Department of Health, Saint Paul, Minnesota, USA (J.T. Wotton); Maine Department of Health and Human Services, Augusta, Maine, USA (S. Robinson); Washington State Department of Health, Shoreline, Washington, USA (B. Leader); Texas Department of State Health Services, Austin, Texas, USA (M. Thompson); and Delaware Health and Social Services, New Castle, Delaware, USA (M. Shannon)
1Additional members of the US Antiviral Resistance Surveillance Working Group who contributed data are listed at the end of this article.
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