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H1N1 Tamiflu Resistance at 100% in South Africa
Recombinomics Commentary 23:50
July 18, 2008

In South Africa, a total of 90 A(H1N1) viruses have been isolated during the 2008 influenza season to date, and all of the 23 influenza A(H1N1) viruses tested by the WHO collaborating Centres in London and Melbourne were found to have resistance to oseltamivir by neuraminidase enzyme-inhibition assay.

None of these patients were receiving oseltamivir at the time of sampling, and no unusual clinical feature or underlying conditions have been found.

To date, preliminary test results show that the viruses carry the specific neuraminidase mutation (H274Y) that confers oseltamivir resistance in N1

From Chile, three of the 24 A(H1N1) viruses tested showed the specific neuraminidase mutation (H274Y).

The above comments from the WHO update on oseltamivir (Tamiflu) resistance indicate the frequency has now reached 100% in South Africa (based on the first 23 H1N1 samples tested).  In the southern hemisphere, the 2008 flu season is ongoing.  Consequently, Chile is also reporting H274Y in the current season.  These isolates are almost certainly Brisbane/59 (clade 2B), which has been linked to the vast majority of Tamiflu resistant isolates from the 2007/2008 season.

Earlier positives from last season were on New Caledonia (clade 1) genetic backgrounds in the United States and Hong Kong (clade 2C) backgrounds in China.  This season there have been multiple introductions of H274Y onto the Brisbane (clade 2B). 

The expansion of H274Y has been facilitated by the vaccine mismatch, which targeted Solomon Islands (clade 2A) this season.  There is no evidence for any clade 2A in circulation this season.

The expansion of H274Y via the Brisbane strain is cause for concern.  It has now reached 100% in South Africa, which represents a growing reservoir of H274Y, which can clearly jump from one H1N1 clade to another, which is most easily explained by homologous recombination.

This polymorphism is identical to the H274Y on H5N1, suggesting that oseltamivir will have limited value for blunting an H5N1 pandmeic.

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