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 [(H3N2)v] viruses. The following case definitions are for the purpose of investigations of suspected, probable, and confirmed cases of influenza A (H3N2)v virus infection. CDC is requesting notification of all suspected and probable cases of influenza A (H3N2)v virus infection within 24 hours of identification. When possible, state health departments are encouraged to investigate suspected cases of influenza A (H3N2)v virus infection further to determine case status.

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

A suspected case of influenza A (H3N2)v virus infection is defined as acute respiratory illness (ARI) in a person who:

• Is epidemiologically linked to a confirmed case of influenza A (H3N2)v virus infection (see below).

OR

• Has had illness onset within 7 days of swine exposure.

A probable case of influenza A (H3N2)v virus infection is defined as ARI in a person who:

• Had a specimen tested using the CDC Human Influenza Virus real-time reverse-transcriptase polymerase chain reaction (rRT-PCR) Diagnostic Panel and is positive for InfA, pdmInfA, and H3; and negative for InfB, pdmH1, and H1.

OR

• Is epidemiologically linked to a confirmed case of influenza A (H3N2)v, and for whom results of influenza diagnostic testing of an appropriate respiratory specimen are positive for influenza A (H3) or positive for influenza A (no subtype tested or detected).

A confirmed case of influenza A (H3N2)v virus infection is defined as influenza A (H3N2)v virus infection in a person that is laboratory-confirmed at CDC.