2023 - 2024 Flu Activity Report

ILI “Influenza like Illness” is defined as fever ≥ 100°F with a cough and/or sore throat.

Specimens included within this report are tested by WHO/NREVSS collaborating laboratories, ACL, and IDPH laboratories.

For the purpose of diagnosis, influenza can be diagnosed by using the following tests: reverse transcription polymerase chain reaction (RT-PCR), viral culture, Immunofluorescence [Direct Fluorescent Antibody (DFA) or Indirect Fluorescent Antibody (IFA) Staining], Enzyme Immuno Assay (EIA) or any rapid diagnostic test. Sensitivities of rapid diagnostic tests are approximately 50-70% when compared with viral culture or reverse transcription polymerase chain reaction (RT-PCR), and specificities of rapid diagnostic tests for influenza are approximately 90-95%. False-positive (and true-negative) results are more likely to occur when disease prevalence in the community is low, which is generally at the beginning and end of the influenza seasons. False-negative (and true-positive) results are more likely to occur when disease prevalence is high in the community, which is typically at the height of the influenza season.

Beginning with week 44 (October 29, 2017), 2017-2018 influenza-like illness (ILI) data from Chicago sentinel providers are included; previous seasonal ILI data excluded Chicago participants.

The Illinois Department of Public Health collects emergency department visits daily from all acute care hospitals in Illinois for syndromic surveillance reporting. In this report, visits are classified as influenza-like illness (ILI) based on chief complaints that include terms for influenza, or fever and cough or sore throat, similar to the ILINet definition while excluding terms unrelated to ILI presentation, such as a flu shot or stomach flu. Data presented here should be comparable to the results from ILI sentinel reporting, and for some hospital emergency departments this data is incorporated into ILINet directly to improve coverage and representation in regions of Illinois with few sentinel providers. For more information about the syndromic surveillance in Illinois visit syndromic-surveillance. The IDPH syndromic surveillance system is supported by the Centers for Disease Control and Prevention’s National Syndromic Surveillance Program, which is where data on the HHS Region 5 and National ILI trends are derived. Visit https://www.cste.org/page/nsspcop for more information.