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. The following definitions are for the purpose of investigations of confirmed and probable cases, and cases of influenza A (H3N2)v virus infection under investigation. CDC is requesting notification of all confirmed and probable 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

Influenza A (H3N2) variant Virus Case Definition

Confirmed: Influenza A (H3N2) variant (H3N2v) virus infection with laboratory confirmation by:

• Genetic sequencing performed at the CDC Influenza Division Laboratory

OR

• Presumptive positive results consistent with H3N2v (Influenza A/H3 and pdmInfA positive, H1 AND pdmH1 negative) using CDC Flu rRT-PCR Dx Panel.

OR

• A 4-fold rise in strain specific serum antibodies with specimens drawn 2 weeks apart.

Probable: A case meeting clinical criteria* and criteria for epidemiologic linkage†, but for which no confirmatory laboratory testing for influenza virus infection has been performed, or for which test results do not provide a sufficient level of detail to confirm H3N2v (e.g., a positive rapid influenza diagnostic test).

Case Under Investigation: A case meeting the clinical criteria* but not criteria for epidemiologic linkage and is pending laboratory confirmation.

* Clinical criteria: Illness compatible with influenza virus infection: fever ≥100°F with cough and/or sore throat

† Criteria for Epidemiologic Linkage:

• Contact with one or more confirmed cases of H3N2v virus infection or attendance at an event where confirmed cases have been identified.

AND

• Transmission of H3N2v virus by the usual modes of respiratory pathogen transmission is plausible.

A case may be considered epidemiologically linked to a laboratory-confirmed case if at least one case in the chain of transmission is laboratory-confirmed.