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H1N1 Tamflu Resistance Is Widespread In the United States
Recombinomics Commentary 23:55
January 5, 2009

The limited number and geographic diversity of specimens tested for antiviral resistance, as well as the uncertainty regarding which influenza virus types or subtypes will predominate during the season, make it too early to make an accurate determination of the prevalence of influenza viruses resistant to oseltamivir nationally or regionally at this time. CDC has solicited a representative sample of viruses from WHO collaborating laboratories in the United States, and more specimens are expected as influenza activity increases.

The above comments are from the week 52 CDC flu report, but the picture of antiviral resistance in the United States is becoming increasingly clear.  The level of Tamiflu resistance in H1N1 is near 100% (70/71 isolates have H274Y), while H3N2 is 100% adamantine resistant.  The only reported Tamiflu sensitive isolate since October 1 was one isolate among the first two reported early in the season.  Since that initial report, 69/69 H1N1 isolates have had H274Y.

Although most of the early H1N1 isolates were from Texas and Hawaii, the week 52 report shows more widespread dominance of H1N1.  Among the sub-typed influenza A 301/334 are H1N1.  The only reported region with more H3N2 than H1N1 is the mountain states where 21/35 isolates are H3N2.  All other regions are dominated by H1N1, although there was only one isolate (H1N1) in the southeast and only four in New England (three are H1N1).  In West South Central, which includes Texas, all 55 isolates are H1N1 and the Pacific< which includes Hawaii, 171/177 are H1N1.  However, all isolates in the West North Central (6) and South Atlantic (18) are H1N1, while Mid-Atlantic (20/23) and East North Central (13/15) are heavily H1N1, so it is likely that these regions are represented in the H1N1 testing, and all tested H1N1 has H274Y.

While it is possible that H3N2 levels will increase later in the season, as they did last season, right now the vast majority of infleunza A throughout the United States is H1N1 and Tamiflu resistant.  Moreover, similar levels of resistance in H1N1 have been reported for Canada and Europe, so emergence of sensitive from H1N1 from those areas is unlikely.

Tamiflu sensitive H1N1 was widespread in Asia last season, but the clade 2C that was present in western states, including Hawaii, last season, is absent this season.  All H1N1 since October 1, 2008 appear to be clade 2B because reported amantadine resistance in H1N1 is zero, and last season clade 2C was 100% amantadine resistant.  The only reported amantadine resistance in US isolates were from Missouri and California, but the isolates were collected in August and September and prior to the October 1 start date used in the weekly report.

The current high levels of H1N1 in the US simplify antiviral treatment, but choices will become more difficult if H3N2 levels rise, because many states do not report sub-type frequencies.

Increased surveillance and sub-type analysis at state lab reports would be useful.

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