Interim Guidance on Case Definitions to be Used For Investigations of Influenza A (H3N2)v Virus Cases

This document provides updated interim guidance for state and local health departments conducting investigations of infections with influenza A (H3N2) variant (H3N2v) viruses. Influenza viruses that typically infect swine are referred to as “variant” when they infect humans. The following definitions are for the purpose of investigations of confirmed cases and cases of influenza A (H3N2)v virus infection under investigation. CDC is requesting notification of all confirmed cases of influenza A (H3N2)v virus infection within 24 hours of identification. When possible, state health departments are encouraged to investigate all potential cases of influenza A (H3N2)v virus infection further to determine case status.

Case Definitions for Infection with Influenza A (H3N2)v Virus

Confirmed: Influenza A (H3N2)v virus infection in a patient with laboratory confirmation by:

  • Reverse-transcription polymerase chain reaction (RT-PCR) testing or genetic sequencing results positive for influenza A (H3N2)v virus at the CDC Influenza Division Laboratory


  • RT-PCR testing results at a public health laboratory consistent with influenza A (H3N2)v virus using a CDC-approved assay (for example, InfA, H3, and pdmInfA positive results, and H1 and pdmH1 negative results using the CDC Flu rRT-PCR Dx Panel)1. Please see Interim Guidance on Specimen Collection, Processing, and Testing for Patients with Suspect Influenza A (H3N2)v Virus Infection for additional guidance on testing for H3N2v viruses.

Case Under Investigation: Illness compatible with influenza2 in a patient meeting at least one of the epidemiologic criteria below for whom laboratory confirmation is not known or pending, or for whom test results do not provide a sufficient level of detail to confirm influenza A (H3N2)v virus (e.g., a positive rapid influenza diagnostic test).

  • Recent close contact3 (within 7 days of illness onset) with confirmed cases of influenza A (H3N2)v virus infection


  • Recent contact (within 7 days of illness onset) with swine or recent attendance at an event (such as an agricultural fair) where swine were present. Contact with swine may be direct contact (i.e., touching or handling a pig) or indirect contact (coming within about 6 feet (2 meters) of a pig without known direct contact).

1 This laboratory result is reportable as “presumptive positive” for influenza A (H3N2v) as specified in the CDC Flu rRT-PCR Dx Panel in vitro diagnostic (IVD) package insert. Although State public health laboratories are able to report and act upon this result, all specimens with “presumptive positive” results should be sent to CDC for additional testing. Please see Data Interpretation Update to the CDC Flu rRT-PCR Dx Panel [27 KB, 1 page] for additional guidance on interpretation of the CDC Flu rRT-PCR Dx Panel.

2 Illness compatible with influenza may present as influenza-like illness (ILI) [fever ≥100°F plus cough or sore throat]. Note that influenza may not cause fever in all patients (especially in patients under 5 years of age, over 65 years of age, or patients with immune-suppression), and the absence of fever should not supersede clinical judgment when evaluating a patient for illness compatible with influenza.

3 Close contact may be regarded as coming within about 6 feet (2 meters) of a confirmed case while the case was ill (beginning 1 day prior to symptom onset and continuing until resolution of illness). This includes healthcare personnel providing care for a confirmed case, family members of a confirmed case, persons who lived with or stayed overnight with a confirmed case, and others who have had similar close physical contact.