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West Virginia trH3N2 Cluster Raises Detection Concens
Recombinomics Commentary 21:00
December 21, 2011

The confirmation of the trH3N2 cluster was expected because the second case was epidemiologically linked to the confirmed case, A/West Virginia/06/2011, and the sample gave a weak influenza A signal.  Consequently, it was originally listed as influenza A.  Today the designation was changed to trH3N2 because of confirmation by the CDC, which will be described in the week 50 FluView.  However, the absence of an epidemiological link would have probably led to a designation as a seasonal H3 or an inconclusive/negative.

The data in West Virginia was remarkably similar to the results in Maine.  One case, A/Maine/06/2011, was H3N2pdm11was collected in week 41 and quickly confirmed, in part due to activity for the H1N1pdm09 NP PCR target.  However, the second case in Maine, A/Maine/07/2011, had a low level or RNA leading to an H3 weak positive, and a negative for the NP target.  Thus, it would have also been classified as seasonal H3 without the swine exposure designation and attendance at the same Fryeburg fair as the first case.  However, the fair and associated pig scramble were two week prior to disease onset, and therefore not related to the H3N2pdm11 infection, and the close sequence relationship between the two Maine isolates strongly suggest this virus was transmitting human to human in Maine (where no SOIV’s have been reported in swine).
Thus, the second case in both states was identified as novel by CDC sequencing, not PCR testing, and highlights the need for aggressive surveillance, since both states have each only reported two influenza cases in the 2011/2012 season, and all four case were novel.  The two cases in West Virginia were epidemiologically linked, and not linked to swine, representing the first report of cluster involving distinct trH3N2 constellations, which are in addition to the trH1N2 case, A/Minnesota/19/2011, in Minnesota, which was a suspect cluster linked to a symptomatic but untested contact due to delays associated with routine testing of an isolate.

Thus, the three clusters, couple with the weak signals for the second case in Maine and West Virginia, highlight the need for enhanced surveillance and aggressive sequencing, especially in samples that produce weak signals, in part because the PCR testing is indirect since the targets are not from trH3N2 or trH1N2 sequences.

Both the CDC and WHO have requested an increase in surveillance and recommended testing at regional influenza centers, but the significance of three confirmed or suspect clusters involving three distinct constellations of triple reassortant genes, has been largely lost on the public, due in part to prior attempts to link these cases to swine.

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