Thursday, July 9, 2009

H1N1 Influenza A - update

Sent: Wednesday, July 08, 2009 2:49 PM
Subject: Centers for Disease Control and Prevention (CDC) Update

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July 7, 2009


· Three novel H1N1 influenza viruses that are resistant to the antiviral drug oseltamivir have been detected from 3 countries.

· These rare instances of oseltamivir resistant novel H1N1 influenza viruses remain isolated findings at this time.

· The oseltamivir resistant viruses identified have been sensitive (susceptible) to zanamivir.

· There is no evidence of genetic reassortment with seasonal H1 viruses among the three cases of oseltamivir resistant novel H1N1 influenza viruses.

· At this time, WHO and CDC do not recommend any changes in antiviral guidance.

· The few people who have been infected with oseltamivir-resistant viruses have had illness similar to that caused by oseltamivir-sensitive viruses. Illness has not been more severe, and oseltamivir-resistant viruses have not been identified among close contacts.

· Surveillance for the detection of antiviral resistance in novel H1N1 influenza is ongoing among domestic and international isolates submitted to CDC.

· There are two influenza antiviral medications recommended for use against novel H1N1 influenza. These are oseltamivir (trade name Tamiflu ®) and zanamivir (trade name Relenza ®). Either medication can be used.

· Highest priority should be placed on treating patients hospitalized with influenza or those who are ill with influenza who have an age or medical factor placing them at higher risk for more severe illness or influenza-related complications.

· See Interim Guidance on Antiviral Recommendations for Patients with Novel Influenza A (H1N1) Virus Infection and Their Close Contacts for more information.


· Influenza viruses can develop resistance to antiviral medications.

· Antiviral resistance means that a virus has changed in such a way that the antiviral drug is less effective in treating or preventing illnesses caused by the virus.

· Influenza viruses constantly change as the virus makes copies of itself. Some changes can result in the viruses being resistant to one or more of the antiviral drugs that are used to treat or prevent influenza.

· Antiviral resistance is detected through laboratory testing.

· Additional cases of antiviral resistance are likely to be detected.

· CDC and its WHO partners continue to conduct surveillance for antiviral resistance. The data indicate that the prevalence of oseltamivir resistant viruses is low. Among 202 novel influenza A (H1N1) viruses from the United States tested by CDC this year, none have been resistant to oseltamivir.

· Information on resistance of influenza viruses to the four antiviral medications is updated weekly on the CDC FluView surveillance report which is found at:


· Influenza antiviral medications are prescription medicines (pills, liquid or an inhaled powder) with activity against influenza viruses, including novel H1N1 influenza viruses.

· Antiviral drugs work by decreasing the spread of flu viruses in the respiratory tract.

· Influenza antiviral medications work best when started soon after illness onset (within 2 days) , but treatment with antiviral drugs should still be considered after 48 hours of symptom onset, particularly for hospitalized patients or people at high risk for influenza-related complications.

· There are four influenza antiviral medications approved for use in the United States . The four antiviral drugs are oseltamivir (brand name Tamiflu ®) ; zanamivir (brand name Relenza ®) ; amantadine (Symmetrel®, generic) ; and rimantadine (Flumadine®, generic) .

o This novel (H1N1) influenza virus is sensitive (susceptible) to the neuraminidase inhibitor antiviral medications, zanamivir and oseltamivir (other than the three viruses recently identified). It is resistant to the adamantane antiviral medications, amantadine and rimantadine.

· Most persons with novel H1N1 influenza have had mild illness lasting several days and have recovered without need for antiviral treatment. Treatment is generally reserved for patients hospitalized with influenza or those who are ill with influenza who have an age or medical factor placing them at higher risk for more severe illness or influenza-related complications.

· Use of antiviral drugs to prevent illness (chemoprophylaxis) is usually reserved for certain specific situations. Widespread use of antiviral medications for chemoprophylaxis is not encouraged as injudicious use of antiviral drugs might be a factor in causing more viruses to become resistant.

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