Outbreak of variant influenza A(H3N2) virus in the United States

Abstract

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 multistate outbreak of an influenza A(H3N2) variant virus (H3N2v) 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 transmission if swine contact was not identified within 4 days prior to illness onset.

Results: From 9 July to 7 September 2012, we identified 306 cases of H3N2v in 10 states. The median age of all patients was 7 years. Commonly reported signs and symptoms included fever (98%), cough (85%), and fatigue (83%). Sixteen patients (5.2%) were hospitalized, and 1 fatal case was identified. The majority of those infected 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. Viruses recovered from patients were 93%-100% identical and similar to viruses recovered from previous cases of H3N2v. All H3N2v viruses examined were susceptible to oseltamivir and zanamivir and resistant to adamantane antiviral medications.

Conclusions: In a large outbreak of variant influenza, the majority of infected persons reported exposures, suggesting that swine contact at an agricultural fair was a risk for H3N2v 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.

Keywords: influenza; outbreak; pandemic; variant influenza.

Figure 1.

Epidemic curve of confirmed cases of influenza A(H3N2) variant virus infection in the United States, July–September 2012 (N = 306) and estimate of the number of fairs occurring in the United States. Data on fairs were obtained from a directory of registered fairs from the International Association of Fairs and Expositions, which includes information on the location and date of voluntarily registered fairs (approximately 40% of the estimated 3000 fairs held annually in the United States). Estimates of the number of total fairs occurring each week were obtained by extrapolating the time distribution of registered fairs to the estimated number of total fairs.

Figure 2.

Geographic distribution of influenza A(H3N2) variant virus cases, by county, United States, July–September 2012, and number of pigs by county (2007). Number of pigs by county obtained from the 2007 Census of Agriculture, United States Department of Agriculture, National Agricultural Statistics Service.