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Iowa trH3N2 Cluster of Five Raises Concerns
During her illness days two and three, patient A was in contact with her father, who subsequently developed ILI 2 days after his most recent contact with patient A.
Patient B is a previously healthy male child who developed ILI 2 days after patient A's first day of illness. He is the sibling of patient B.
The above detail from the CDC’s early release MMWR, "Limited Human-to-Human Transmission of Novel Influenza A (H3N2) Virus — Iowa, November 2011", strongly confirms that the two index case (A/Iowa/07/2011) contacts (brother and father), who developed ILI shortly before or after the index case were also infected with trH3N2, although not lab confirmed.
The detail signals efficient transmission, which includes an adult. Moreover, the transmission chain was at least H2H2H, and if the index case was infected by her brother, the chain is H2H2H2H, involving family members, a school contact (A/Iowa/08/2011), and his brother (A/Iowa/09/2011).
This efficient transmission supports human transmission in the earlier trH3N2 cases, which have loose direct or indirect casual exposure with asymptomatic swine, none of which have been shown to be infected by any SOIV (one case visited a fair with symptomatic swine.
All 10 of the 2011 cases were infected with a novel trH3N2 which had an M gene from H1N1pnd09. All 8 genes shared the same lineages, which trace back to human isolates of trH3N2. This constellation and lineage has only been reported in one recent (Sept 13) isolate from a pig in New York, A/swine/NY/A01104005/2011.
However, the three sequences from the Iowa cluster acquired PB2 M315T, which is in virtually all H1N1pdm09, raising concerns of further adaptation to humans and/or efficient transmission.
The CDC has requested samples and noted technical difficulties linked to distinguishing seasonal H3N2 from trH3N2, and requested samples, although “swine exposure” for prior cases was mentioned, which may limit and bias future samples and testing.