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Paradigm Shift Intervention Monitoring
Sequences from one of the recently
described Chihuahua isolates, A/Texas/08/2011, demonstrates
reassortment. The HA sequence is identical to other recently
released Chihuahua isolates from the United States (A/Texas/07/2011,
A/Oregon/03/2011, A/Maryland/04/2011, A/Pennsylvania/04/2011, A/New
Jersey/05/2011), but the NA sequence is closely related to sequences
from the S188T sub-clade (A/Mississippi/04/2011, A/Wisconsin/16/2011,
A/North Carolina/09/2011, A/North Carolina/06/2011,
A/Connecticut/04/2011, as well as 2010 Karaj and Tehran isolates from
The reassortment between two dominant co-circulating sub-clades has parallel with the fixing of Tamiflu resistance (H274Y) in 2008, when there were two co-circulating seasonal H1N1 sub-clades. Clade 2B (Brisbane/59/2007) was widespread in North America and Europe, while clade 2C (Hong Kong/2562/2006) was widespread in Asia and western United States. Hong Kong had both sub-clades and reassortants with HA from clade 2C and NA from clade 2B were isolated, which had H274Y.
In addition to exchanges of genetic information by reassortment, there were additional exchanges via recombination. H274Y jumped from one gentic background to another. Similarly, clade 2B acquired clade 2C polymorphisms.
Similar exchanges are being seen in pandemic H1N1. S188T isolates have acquired S165N or A189T from the Chihuahua sub-clade, and H274Y in Delaware and Maryland is jumping from one genetic background to another.
These parallels increase concerns that Tamiflu resistance (H274Y) will become increasingly common, as was seen in 2008 and 2009.