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H3N2v H2H Clonal Expansion Raises School Concerns
Recombinomics Commentary 23:55
August 31, 2012

In the U.S., influenza surveillance systems that normally start up in October have been activated early to look for cases, Finelli said.

She said the CDC is concerned that with the resumption of school and the impending change in the weather this virus could become more transmissible among people.

So far only a few instances of person-to-person spread have been seen with H3N2v. But Finelli noted that late last year there was some human-to-human transmission in Iowa and West Virginia. While the numbers last year weren’t high, she said the CDC is worried that pattern might be repeated this year.

The above comments from a Helen Branswell piece notes the two H3N2v outbreaks at the end of 2011 that did not have swine exposure. 

In the Iowa cluster three cases were confirmed (A/Iowa/07/2011, A/Iowa/08/2011, A/Iowa/2011).  All three sequences were virtually identical confirming human to human transmission between classmates at the daycare center.  In addition to the three confirmed cases, the father and brother of the index were also symptomatic but not tested.  This cluster however, involved the last human cases involving the sub-clade linked to the seven prior cases.  It is currently circulating in swine, and the USDA has identified 26 matches, but the last human cases involving swine exposure were in October, 2011.

In contrast, the West Virginia cluster involved a novel sub-clade, which had an N2 gene that was a different lineage (found in swine H3N2 instead of swine H1N2).  Two confirmed classmates at a Mineral County day care center were confirmed (A/West Virginia/06/2011 and A/West Virginia/07/2011), bit the disease onset dates were two weeks apart and an epidemiological study identified ILI in 23 out of 70 contacts of the index case (patient zero) indicating the H3N2 had been transmitting at the day care center for a month.

This sub-clade has only been found in two swine isolates in 2012 (North Carolina and Indiana), but all 35 sets of sequences from human cases match the West Virginia sub-clade.  Moreover, the clusters in 2012 were larger that the day care center.  The outbreak at the Gallia Junior County fair in Ohio involved hundreds of attendees with ILI (including 12 H3N2v confirmed cases), while the Monroe County Fair in Indiana had 61 confirmed H3N2v cases.

Thus, the West Virginia sub-clade has transmitted efficiently in humans, and outbreaks in elementary schools will be common.

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