Interim Guidance on Case Definitions to be Used For Investigations of Novel Influenza A (H1N1) Cases*

June 1, 2009 12:15 PM ET

This document provides interim guidance for state and local health departments conducting investigations of human cases of novel influenza A (H1N1) virus (formerly referred to as swine-origin influenza virus). The following case definitions are for the purpose of public health investigations of suspected, probable, and confirmed cases of swine-origin influenza A (H1N1) virus infection.This guidance has been updated to include the specific case definition of ILI that is used for seasonal influenza surveillance purposes.

Influenza-like-illness (ILI) is defined as fever (temperature of 100°F [37.8°C] or greater) and a cough and/or a sore throat in the absence of a KNOWN cause other than influenza.

Case Definitions for Infection with Novel Influenza A (H1N1) Virus

A confirmed case of novel influenza A (H1N1) virus infection is defined as a person with an influenza-like illness with laboratory confirmed novel influenza A (H1N1) virus infection by one or more of the following tests:

1. real-time RT-PCR
2. viral culture
   

A probable case of novel influenza A (H1N1) virus infection is defined as a person with an influenza-like-illness who is

• positive for influenza A, but negative for human H1 and H3 by influenza RT_PCR

Optional

A suspected case of novel influenza A (H1N1) virus infection is defined as a person who does not meet the confirmed or probable case definition, and is not novel H1N1 test negative, and is/has:

• a previously healthy person < 65 years hospitalized for ILI

OR

• ILI and resides in a state without confirmed cases, but has traveled to a state or country where there are one or more confirmed or probable cases

OR

• ILI and has an epidemiologic link in the past 7 days to a confirmed case or probable case