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H3N2 Dominance In US Raises trH3N2 Pandemic Concerns
Recombinomics Commentary 12:45
September 10, 2011

The figure below represents the CDC’s week 35 FluView graph of serotypes.  In addition to the unsubtypable for week 33, which was added to the underlying data a week ago, the updated data includes an unsubtypable for week 34 as well as two additional H3N2 isolates.  These updates after the initial report are not unusual, but the increases in H3N2 raise concerns that many or most of these isolates being reported as seasonal H3N2 are actually trH3N2.

Week 34 Sub-types
In the past three weeks H3N2 has become the dominant serotype, as represented by the seasonal H3N2 column as well as the unsubtypables, which are clearly trH3N2 isolates which correlate with the reported case from Washington County, Pennsylvania.
As was reported for the delayed reporting of an early trH3N2 isolate from Pennsylvania, A/Pennsylvania/40/2010, trH3N2 isolates frequently subtype as seasonal H3N2 because the H3 and H2 trace back to reasortants involving seasonal H3N2.  Thus, the human origin of these genes generates a positive result when assayed by H3 and N2 typing reagents, which target human determinants.

However, the two recent unsubtypables indicate that the 2011 isolates from Pennsylvania fail to type, leading to a sub-typing profile of influenza A positive but H3 and H1 negative for trH3N2, similar to results for pandemic H1N1, which has a swine H1 and N1.  Thus, the recent evolution of H3 has moved the trH3 away from recognition by human H3 reagents, but other isolates, like A/Pennsylvania/40/2010 will return a seasonal H3N2 result.  Thus, many of the recently reported seasonal H3N2 isolates may be trH3N2.

These newly reported H3N2 isolates, as well as those that are influenza A positive, should be PCR tested, which is also true for early H3N2 isolates, including the large number of unsubtypables reported by Pennsylvania in the 2010/2011 season.

More detail on the recent H3N2 isolates classified as seasonal H3N2 would be useful.

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