US Reports Outbreak of Variant Influenza A (H3N2v) Virus

US - Variant influenza virus infections are rare but may have pandemic potential if person-to-person transmission is efficient. Researchers from the National Center for Immunization and Respiratory Disease describe the epidemiology of a multi-state outbreak of an influenza A H3N2v virus first identified in 2011.
calendar icon 8 November 2013
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In the journal, Clinical Infectious Diseases, researchers from the US National Center for Immunization and Respiratory Disease led by M.A. Jhung identified laboratory-confirmed cases of H3N2v and used a standard case report form to characterize illness and exposures. They considered illness to result from person-to-person H3N2v virus transmission if swine contact was not identified within four days prior to illness onset.


From 9 July - 7 September 2012, they identified 306 cases of H3N2v in ten states. The median age of all cases was six years. Commonly reported signs and symptoms included fever (98 per cent), cough (84 per cent), and fatigue (83 per cent). Sixteen cases (5.2 per cent) were hospitalized, and one fatal case was identified. The majority of cases reported agricultural fair attendance (93 per cent) and/or contact with swine (95 per cent) prior to illness.

The researchers identified 15 cases of possible person-to-person transmission of H3N2v virus. Viruses recovered from cases were 93 per cent to 100 per cent identical and similar to viruses recovered from previous cases of H3N2v. All H3N2v viruses examined were susceptible to the oseltamivir and zanamivir and resistant to adamantane antiviral medications.


In a large outbreak of variant influenza, the majority of cases reported exposures suggesting swine contact at an agricultural fair was a risk for H3N2v virus infection. The researchers identified limited person-to-person H3N2v virus transmission, but found no evidence of efficient or sustained person-to-person transmission. According to the researchers, fair managers and attendees should be aware of the risk of swine-to-human transmission of influenza viruses in these settings.


Jhung MA, Epperson S, Biggerstaff M, Allen D, Balish A, Barnes N, Beaudoin A, Berman L, Bidol S, Blanton L, Blythe D, Brammer L, D'Mello T, Danila R, Davis W, de Fijter S, Diorio M, Durand LO, Emery S, Fowler B, Garten R, Grant Y, Greenbaum A, Gubareva L, Havers F, Haupt T, House J, Ibrahim S, Jiang V, Jain S, Jernigan D, Kazmierczak J, Klimov A, Lindstrom S, Longenberger A, Lucas P, Lynfield R, McMorrow M, Moll M, Morin C, Ostroff S, Page SL, Park SY, Peters S, Quinn C, Reed C, Richards S, Scheftel J, Simwale O, Shu B, Soyemi K, Stauffer J, Steffens C, Su S, Torso L, Uyeki TM, Vetter S, Villanueva J, Wong KK, Shaw M, Bresee JS, Cox N, Finelli L; Outbreak of Variant Influenza A (H3N2v) Virus in the United States; Clin Infect Dis. 2013 Sep 24. [Epub ahead of print]

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