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H1N1 Tamiflu Resistance in the United States Increases to 98%
Recombinomics Commentary 13:16
December 15, 2008

Forty-five of 46 influenza A (H1N1) viruses tested were resistant to oseltamivir

Twenty-five influenza A (H1N1) and five influenza A (H3N2) viruses were tested for adamantane resistance. All influenza A (H1N1) viruses were sensitive to the adamantanes. All influenza A (H3N2) viruses tested were resistant to the adamantanes.

Limited data on antiviral resistance, as well as the uncertainty regarding which influenza virus types or subtypes will circulate during the season, make it too early to make an accurate determination of the prevalence of influenza viruses resistance to oseltamivir or the adamantanes (amantadine and rimantadine) nationally or regionally at this time.

The above comments from the week 49 report on influenza in the United States extend and confirm earlier data, and once again highlight the mismatch between the frequencies of antiviral resistance in the US, and the CDC recommendations which discourage use of amantadines and encourage oseltamivir.

Currently amantadine resiatnce in H3N2 remains at 100%, while similar frequencies (currently 98%) are found for oseltamivir resistance in H1N1.

Most of the influenza in the US is influenza A 408/507, and most of the influenza A is H1N1 (157/175).  Last season the level of osletamivir resistance in the US was closer to 10% because clade 2C was in circulation, which had no oseltamivir resistance, but was 100% amantadine resistant.  Moreover, most of the oseltamivir resistance (H274Y) in H1N1 was limited to a specific 2B sub-clade.

However, this 2B (Brisbane/59) sub-clade achieved dominance in the southern hemisphere in 2008, and now represents almost all clade 2B in North America and Europe.  Thus, now all tested H1N1 in the US and Europe is amantadine sensitive and oseltamivir resistance.

H3N2 has maintained its resistance to amantadine, and in Europe H3N2 is dominant.  However, in the US H1N1 is dominant, so the level of oseltamivir resistance is much higher than amantadine resistance, and the CDC recommendations remain mismatched.

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