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|Paradigm Shift Intervention Monitoring||Audio: Jan28 Apr21 Sep22
Tamiflu Resistance in United States Near 100%
Six influenza A (H1N1) and four influenza A (H3N2) viruses were tested for adamantane resistance. All influenza A (H1N1) viruses were sensitive to adamantanes and all influenza A (H3N2) viruses tested were resistant to adamantanes.
The above comments from this week’s weekly report from the CDC confirm that most H1N1 in the Unites States this season will have H274Y and be oseltamivir resistant. Thus far, most influenza in the United States has been H1N1. 243 of 309 characterized isolates have been influenza A and serotyping of Influenza A has been overwhelmingly H1N1 (101 H1N1, 14 H3N2). This ratio has increased in the latest report and the majority of cases have been in the Pacific region, with highest activity in Hawaii.
Last season H1N1 in Hawaii included clade 2B (Brisbane/59) and clade 2C (Hong Kong). H274Y was reported at a low level, but included the earliest samples that were H274Y positive, and all clade 2C samples were amantadine resistant, consistent with reports on clade 2C isolates in Asia. Thus, last season all amantadine resistant H1N1 was limited to clade 2C. However, as noted above, only six of the 21 H1N1 isolates this season have been tested for amantadine resistance, so it remains unclear if any samples are resistant to both oseltamivir and amantadine. Isolates collected over the summer from Hong Kong were oseltamivir resistant clade 2C raising concerns that this season H274Y would be in clade 2B and clade 2C. Testing of all 21 H1N1 samples for amantadine resistance would add insight to the levels of H274Y in clade 2C in the United States.
It is clear that the levels of H274Y in clade 2B will be near 100%. In Europe England is reporting resistance in 14/15 H1N1 isolates and reported levels remain at 100% in Scotland (2/2), and Norway (1/1), which is also true for Canada (1/1). These isolates were all amantadine sensitive, indicating they were clade 2B.
The US is lagging on reports on antiviral resistance. Oseltamivir resistance results on 21/101 H1N1 samples have been released, but only six have been tested for amantadine resistance. Amantadine resistance results would indicate how many samples are clade 2C, since all recent clade 2C isolates in the United States and Asia were Amantadine resistant (S31N). Full antiviral testing of H1N1 isolates would be useful.