Oseltamivir Resistance

Oseltamivir Resistance in Pandemic Influenza A (H1N1) Isolates

On August 14 and 19, 2009, the Centers for Disease Control (CDC) identified oseltamivir resistance in pandemic influenza A (H1N1) isolates from two epidemiologically linked patients in North Carolina. Both developed illness in July while receiving oseltamivir chemoprophylaxis for presumed exposure to an ill person. Their illnesses were mild and resolved without complications. To date, there is no evidence of ongoing transmission of oseltamivir-resistant pandemic influenza A (H1N1) virus. Most circulating virus remains susceptible to oseltamivir and zanamivir, but resistant to amantidine and rimantidine.

On August 14 and 19, 2009, the Centers for Disease Control (CDC) identified oseltamivir resistance in pandemic influenza A (H1N1) isolates from two epidemiologically linked patients in North Carolina. Both developed illness in July while receiving oseltamivir chemoprophylaxis for presumed exposure to an ill person. Their illnesses were mild and resolved without complications. To date, there is no evidence of ongoing transmission of oseltamivir-resistant pandemic influenza A (H1N1) virus. Most circulating virus remains susceptible to oseltamivir and zanamivir, but resistant to amantidine and rimantidine.

In light of these events, the North Carolina Division of Public Health reminds clinicians of the following:

• Antiviral chemoprophylaxis should be used judiciously to decrease opportunities for development of antiviral resistance.
  • Chemoprophylaxis for prevention of illness in healthy children or adults following exposure to ill persons is not indicated.
  • Antiviral chemoprophylaxis may be considered for persons at higher risk from complications due to influenza or for some health care workers with an exposure to influenza secondary to inadequate personal protective equipment.
  • Careful observation for symptoms following an exposure combined with early treatment if symptoms develop could be an appropriate alternative to chemoprophylaxis in some settings, and could reduce the potential for oseltamivir resistance.
  • Development of symptoms while receiving prophylaxis does not necessarily indicate antiviral resistance. 
• Healthy patients with uncomplicated illness need not be treated with antivirals.
• Antiviral treatment with either oseltamivir or zanamivir is recommended for all patients with confirmed, probable or suspected cases of pandemic influenza A (H1N1) virus infection who are hospitalized or who are at higher risk for influenza complications.
  • If antiviral treatment is indicated in a patient whose symptoms developed >48 hours after beginning oseltamivir prophylaxis, consider use of zanamivir. (Note: Zanamivir is not licensed for treatment of children <7 years of age and is contraindicated in persons with underlying airway disease.)
• Isolation of ill persons, good hand and respiratory hygiene, and vaccination (when available) should be the cornerstones of strategies to prevent transmission of pandemic influenza A (H1N1) viruses.

Surveillance for antiviral resistance is ongoing in North Carolina and throughout the U.S. Testing for antiviral resistance in clinical settings is not widely available. If infection with oseltamivir-resistant pandemic influenza A (H1N1) virus is suspected, observe treatment and infection control recommendations above. Testing for antiviral resistance might be available in certain situations after consultation with local and state health departments.

Complete CDC recommendations regarding antiviral treatment and chemoprophylaxis are available at www.cdc.gov/h1n1flu/recommendations.htm.