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Sustained Efficient Human Community Spread of H3N2v
Recombinomics Commentary 12:45
August 14, 2012

Now, for a little context. CDC understands that people are concerned as to the rapid jump in the number of H3N2 cases compared with last week. At this point, there's no evidence of sustained efficient human-to-human spread in the community.

The above comments from the August 9, 2012 telebriefing on H3N2v have striking similarities to WHO comments on the 2009 H1N1pdm09 pandemic when it was spreading in Europe, Asia, and Australia.  The first two cases had been identified in southern California in children who had no contact with swine or each other (samples collected in late March, 2009).  Subsequent sequence data from Mexico and Canada confirmed that the same H1N1pdm09 had spread throughout North America, and the report of 150 symptomatic cases at a Queens high school confirmed that the H1N1pdm09 had spread efficiently in April.

However, although the above events clearly signaled a pandemic, a pandemic was not declared until June 11, 2009 because the community transmission had not been demonstrated in Europe, Asia, or Australia.  The delay in confirmation in community transmission was largely linked to limited and biased testing.  Countries were screening arriving passengers at airports for fever, which led to detection of H1N1pdm09. 

Epidemiological studies focused on these confirmed cases, but H1N1pdm09 contacts were not considered “community transmission” because of linkage to the arriving passengers.

However, the airport screening only detected a small fraction of H1N1pdm09 cases, because half of the infections did not lead to high fevers.  Moreover, those infected 1-2 days prior to flying developed high fevers after passing through screening, while others decreased their fever by using medications to treat their flu symptoms.  Thus, although there was community spread by cases who were not identified in the screenings, the community spread was not initially detected because resources were concentrated on testing airport passengers.

As a result, almost all H1N1[dm09 cases were linked to air travel and flight attendants on the arriving planes, but the H1N1pdm09 cases were not attributed to the airplane or flight attendant linkage, because it was well known that almost all testing was focused on arriving passengers.

This linkage is similar to the current detection of H3N2v at livestock fairs, because those are the cases that are being testing.  Mild ILI cases without direct or indirect swine are not being tested, which leads to heavily biased sampling and an artificial and unscientific linkage, which then leads to more biased testing and more artificial links and an explosion of media myths on H3N2v jumping from swine to humans at agricultural fairs.

The H3N2v detected in July is distinct from the initial H3N2v cases in 2011, and the novel sub-clade was not widely detected in swine prior to the July outbreaks.  Moreover, like the high school in Queens in 2009, the clusters are large.  200 symptomatic cases linked to the Gallia Junior County Fair were tested using an influenza A rapid test, and 69 of the 200 cases were positive.  A subset was sent to the Ohio Department of Health, and thus far H3N2v has been confirmed in 11, which is similar to the initial data on cases at the 2009 Queens high school.

These large clusters, in combination with confirmed cases in 11 counties in Ohio and 18 counties in Indiana, leaves little doubt that there is efficient human-to-human spread in the community.  All sequences released to date match the novel sub-clade detected in the West Virginia day care where there was not direct or indirect swine exposure.

The absence of reported community transmission is linked to biased and limited testing

A refocus on testing ILI cases with no swine contact is long overdue.

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