The WHO alert on two H3N2 triple reassortants in the United States (Illinois – 7 mo old child and Pennsylvania – 46M) raises pandemic concerns. The Pennsylvania case is listed in Friday’s MMWR. It seems likely that the Illinois case is not yet reported in the MMWR (an earlier case, first listed in June, is likely an Iowa case reported in early 2010).
If the two cases in the WHO alert are current and have no contact with each other, then they likely signal the start of another pandemic. H3N2 triple reassortants are commonly found in swine and are widespread in the United States. They have much in common with pandemic H1N1, except the H1 and N1 are replaced by human H3 and human H2. These viruses occasionally jump to humans, but transmission is limited in the human population.
However, reports of two independent cases in the US would be much like the first two cases of pandemic H1N1 in southern California in 2009. The two children were over 100 miles apart and had no known contact with each other or swine. The novel H3N2 cases are over 500 miles apart and swine contact by a 7 month old child seems unlikely.
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