Monday, November 24, 2008

H1N1 Tamiflu Resistance Reporting Delays in North America

Recombinomics Commentary 23:15
November 23, 2008

Based on the level of oseltamivir resistance observed in only one influenza subtype, H1N1, and the persisting high levels of resistance to the adamantanes in H3N2 viruses, CDC continues to recommend the use of oseltamivir and zanamivir for the treatment or prevention of influenza in the United States. Use of amantadine or rimantadine is not recommended.

The above comments from the latest influenza weekly report from the CDC comments on the levels of anti-viral resistance “observed”. However, reports from Europe and North America suggest that virtually all influenza A in the United States this season will be resistant to at least one anti-viral. The resistance rate for H3N2 for amantadines remains at or near 100%. Although test results for this season has been very limited, all H3N2 tested thus far has been resistant to amantadines. Similarly, last season the rate of resistance for the H1N1 sub-clade 2C (Hong Kong) was also 100%. Last season there were two H1N1 sub-clades in circulation in the US, and the rate of Tamiflu resistance for clade 2B (Brisbane/59) was around 10%. However, the level rose to 100% in several countries in the southern hemisphere in the 2008 season, and initial reports from Europe and North America (England, 12/13; Scotland, 2/2; Norway, 1/1; Canada, 1/1; United States, 1/1) suggest the level for clade 2B will be at or near 100% this season.

The numbers “observed” in the United States is low, because the reported number tested is low. In the latest report, the United States has reported 62 H1N1 isolates, but none have been tested for both oseltamivir and amantadine resistance. One isolate collected in September, prior to the official start of this flu season was amantadine resistance, strongly suggesting was clade 2C. One of two isolates tested for Tamiflu resistance this season had H274Y, and it is likely that the other will be amantadine resistant (but test results have not been released) and clade 2C.

Since the level of Tamiflu resistance is expected to be near 100%, it is somewhat surprising that more test results have not been released. The UK has issued an initial report on the 12/13 with H274Y in southwestern England, and has noted that two resistant isolates were also identified in Scotland, indicating the Tamiflu resistance is widespread.

Canada announced the resistance in its first influenza A isolate, but that was over a month ago, and Canada has also not released addition data. Thus, even though the Tamiflu resistance levels of clade 2B are expected to be at or near 100%, each country has only released test results on one clade 2B isolate, and both were resistant, consistent with frequencies reported in Europe this season.

Since most clinicians in North America are unaware of the expectation that the vast majority of Brisbane/59 H1N1 will be Tamiflu resistant, prompt release of test results would be useful.

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