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

Posted August 20, 2012

Summary

This document is an update to interim enhanced surveillance guidance first posted in December 2011. In light of the increasing numbers of cases of H3N2 variant virus (H3N2v) and increasing numbers of states reporting H3N2v cases, states should consider expanding surveillance to include rRT-PCR testing of specimens from ILINet providers statewide, and of specimens collected from people with unusual or severe presentations of ILI. States should also consider collection of specimens from outbreaks of ILI among children in child-care and school settings, since these settings have been associated with person-to-person H3N2v virus transmission in 2011. CDC will continue to evaluate new information as it becomes available and will update this guidance as needed.

Background

Since July 2012, infections with an influenza A (H3N2) variant (H3N2v) virus have been identified in multiple states. The majority of these cases are linked to attendance at agricultural events and exposure to swine. There is no evidence of sustained community-wide transmission. Confirmed H3N2v cases have been identified primarily among children (<18 years of age), and limited serologic studies indicate that children, primarily those younger than 9 years old, have increased susceptibility to these variant influenza viruses. However, some adult H3N2v cases have been identified.

This document provides interim guidance for enhanced surveillance to state and local health departments testing by real-time reverse transcription-polymerase chain reaction (rRT-PCR) for influenza viruses. Use of rRT-PCR testing is important in order to identify which influenza A virus subtypes (e.g. H3N2v viruses versus seasonal H1N1 or H3N2 viruses) are circulating. Due to the ongoing identification of patients with influenza H3N2v virus infection, these guidelines have been developed in an effort to facilitate timely detection and investigation of H3N2v cases by targeting patients with ILI for influenza testing by rRT-PCR.

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

1. All state public health laboratories should use the CDC Human Influenza Real-Time rRT-PCR FLU Diagnostic Panel to screen specimens for InfA, InfB, and RP.
2. 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 Association of Public Health Laboratories [27 KB, 1 page]. Specimens that are presumptive positive for H3N2v virus should be sent to CDC Influenza Division for additional testing as soon as possible.
3. Conduct contact tracing of confirmed and probable influenza A (H3N2)v cases to gather more information about the epidemiology of the virus and modes of transmission. Contact tracing is essential to evaluate potential person-to-person transmission.
4. Currently, while seasonal influenza viruses are circulating at low levels, 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 rRT-PCR testing. States should specifically consider increasing collection of specimens across the state from patients presenting with ILI in the following high priority areas:
   1. All ILINet providers statewide.
   2. ILI outbreaks statewide, particularly among children in child care and school settings, since these settings were associated with person-to-person influenza A (H3N2)v virus transmission in 2011.
   3. Unusual or severe presentations of ILI statewide, including hospitalized persons.
   4. Medically attended ILI and acute respiratory infection (ARI), especially in children in counties or states where confirmed H3N2v cases have occurred.

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