Background. Variant influenza virus infections are rare but may have pandemic potential if person-to-person transmission is efficient. We describe the epidemiology of a multi-state outbreak of an influenza A H3N2v virus first identified in 2011.
Methods. We identified laboratory-confirmed cases of H3N2v and used a standard case report form to characterize illness and exposures. We considered illness to result from person-to-person H3N2v virus transmission if swine contact was not identified within 4 days prior to illness onset.
Results. From July 9—September 7, 2012, we identified 306 cases of H3N2v in ten states. The median age of all cases was 6 years. Commonly reported signs and symptoms included fever (98%), cough (84%), and fatigue (83%). Sixteen cases (5.2%) were hospitalized, and one fatal case was identified. The majority of cases reported agricultural fair attendance (93%) and/or contact with swine (95%) prior to illness. We identified 15 cases of possible person-to-person transmission of H3N2v virus. Viruses recovered from cases were 93% to 100% 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.
Conclusion. 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. We identified limited person-to-person H3N2v virus transmission, but found no evidence of efficient or sustained person-to-person transmission. Fair managers and attendees should be aware of the risk of swine-to-human transmission of influenza viruses in these settings.