Interim Guidance for Influenza Surveillance: Additional Specimen Collection for Detection of Influenza A(H3N2)v Virus Infections

Since August 2011, a number of human infections with an influenza A (H3N2) variant ([H3N2]v)virus have been identified (see Case Count: Detected U.S. Human Infections with H3N2v by State since August 2011). The epidemiology of some of the identified clusters is suggestive of limited person-to-person transmission. Documented illnesses have occurred mostly in children, leading to the concern that children, primarily those in elementary school and younger, may have increased susceptibility to these influenza viruses.

This document provides interim guidance for state and local health departments, hospitals, and clinicians participating in surveillance activities regarding patients targeted for enhanced surveillance and testing by real-time reverse transcriptase-polymerase chain reaction (rRT-PCR) for influenza. Use of rRT-PCR testing is important for surveillance in order to identify which influenza A subtypes (e.g. influenza A(H3N2)v versus seasonal H1N1 or H3N2 viruses) are circulating. Due to the ongoing identification of patients with influenza A(H3N2)v virus infection this fall, these guidelines have been developed in an effort to facilitate timely detection and investigation of cases by targeting patients for influenza testing by rRT-PCR for surveillance.

CDC would like state and local health departments to consider the following recommendations for influenza surveillance and testing.

1. All public health laboratories should use the CDC Human Influenza Real-Time rRT-PCR Diagnostic Panel to screen specimens for InfA, InfB, and RP. Then test all InfA-positive specimens with the CDC Influenza A Subtyping kit using all primer/probe sets: H1, H3, pdmInfA and pdmH1. Detailed guidance for testing can be found in the influenza surveillance diagnostic testing algorithm disseminated recently by APHL.
2. Contact tracing of confirmed, probable, or suspected influenza A(H3N2)v cases (see Case Definitions) should be completed to gather more information about the epidemiology of the virus and modes of transmission.
3. Currently, while there are  low levels of circulating seasonal influenza viruses, CDC recommends increasing collection of specimens from patients with influenza-like illness (ILI) , and having these specimens sent to the state or local laboratory for prioritization for rRT-PCR testing. States should specifically consider increasing collection of specimens from patients presenting with ILI in the following high priority areas:
   1. ILI outbreaks, particularly among children in child-care and school settings, since these have been the settings associated with human-to-human influenza A(H3N2)v virus transmission.
   2. Unusual or severe presentations of ILI, especially among children.
   3. Medically attended ILI and ARI in children under 18 years of age

CDC will continue to evaluate new information as it becomes available and will update this guidance as needed.