H1N1 Tamiflu Resistance in the United States Increases to 96%

Commentary
Recombinomics Commentary 23:38
December 5, 2008

Twenty-four of 25 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.

The above comments from the CDC week 48 report indicates the four most recently tested H1N1 isolates were oseltamivir resistant, raising the frequency in the United States to 96% and extending the streak of Tamiflu resistance to the 23 most recently tested samples.

The latest report significantly increases the number of H1N1 isolates tested for amantadine resistance. Since all are sensitive, it is likely that the H1N1 in the United States is clade 2B (Brisbane/59) and not clade 2C (Hong Kong), which was 100% amantadine resistant last season in the United States.

Thus, clade 2B has become the dominant H1N1 sub-clade in the United States, and has also been the dominant influenza A isolate in the United States since 112/128 subtypes influenza A was H1N1. Similarly, clade 2B is the most common influenza in the United States at this time since 282/365 isolates are influenza A.

The resistance levels in H1N1 in the United States is similar to other countries in North America and Europe, where levels are at or near 100% for Canada (1/1), England (19/20), and Scotland (2/2) and at 100% in Norway and Austria.

At this time H3N2 is the dominant influenza A subtype reported in Europe, but in the United States, the vast majority of influenza A is H1N1, raising concerns regarding the current antiviral recommendations for seasonal flu, which includes oseltamivir.