Investigation of an Outbreak of Variant Influenza A(H3N2) Virus Infection Associated With an Agricultural Fair-Ohio, August 2012

Abstract

Background: In 2012, one third of cases in a multistate outbreak of variant influenza A(H3N2) virus ([H3N2]v) infection occurred in Ohio. We conducted an investigation of (H3N2)v cases associated with agricultural Fair A in Ohio.

Methods: We surveyed Fair A swine exhibitors and their household members. Confirmed cases had influenza-like illness (ILI) and a positive laboratory test for (H3N2)v, and probable cases had ILI. We calculated attack rates. We determined risk factors for infection, using multivariable log-binomial regression.

Results: We identified 20 confirmed and 94 probable cases associated with Fair A. Among 114 cases, the median age was 10 years, there were no hospitalizations or deaths, and 82% had swine exposure. In the exhibitor household cohort of 359 persons (83 households), we identified 6 confirmed cases (2%) and 40 probable cases (11%). An age of <10 years was a significant risk factor (P < .01) for illness. One instance of likely human-to-human transmission was identified.

Conclusions: In this (H3N2)v outbreak, no evidence of sustained human-to-human (H3N2)v transmission was found. Our risk factor analysis contributed to the development of the recommendation that people at increased risk of influenza-associated complications, including children aged <5 years, avoid swine barns at fairs during the 2012 fair season.

Keywords: influenza; outbreak; variant influenza virus.

Figure 1

Variant influenza A(H3N2) virus ([H3N2]v) infection case finding associated with Fair A, Ohio, July–August 2012. Confirmed cases met clinical and exposure criteria and had a positive laboratory test for (H3N2)v. Test results were considered inconclusive if they were negative but samples were collected >4 days after illness onset. Probable cases met clinical criteria (influenza-like illness [ILI]) and exposure criteria (direct or indirect contact with swine, attendance at an event or location where confirmed cases were identified, or contact with a confirmed or probable case) ≤7 days prior to symptom onset but had no testing or inconclusive results of testing for (H3N2)v.

Figure 2

Number of confirmed and probable cases of variant influenza A(H3N2) virus ([H3N2]v) infection, by illness onset date, identified through case finding as associated with Fair A and the total number of confirmed cases for Ohio, July–September 2012. Probable cases met clinical criteria (influenza-like illness) and exposure criteria (direct or indirect contact with swine, attendance at an event or location where confirmed cases were identified, or contact with a confirmed or probable case) ≤7 days prior to symptom onset but had no testing or inconclusive testing for (H3N2)v. Confirmed cases met clinical and exposure criteria and had a positive laboratory test for (H3N2)v. Illness onset date for 1 confirmed case in Ohio was unknown, and this case is not included.

Figure 3

Exhibitor household cohort variant influenza A(H3N2) virus ([H3N2]v) infection investigation, Fair A Ohio, July–August 2012. Confirmed cases met clinical and exposure criteria and had a positive laboratory test for (H3N2)v. Test results were considered inconclusive if they were negative but samples were collected >4 days after illness onset. Probable cases met clinical criteria (influenza-like illness) and exposure criteria (direct or indirect contact with swine, attendance at an event or location where confirmed cases were identified, or contact with a confirmed or probable case) ≤7 days prior to symptom onset but had no testing or inconclusive results of testing for (H3N2)v.