From CDC:
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Information on outpatient visits to health care providers for respiratory illness referred to as influenza-like illness [ILI) is collected through the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet). ILINet consists of outpatient healthcare providers in all 50 states, Puerto Rico, the District of Columbia, and the U.S. Virgin Islands. Approximately 85 million patient visits were reported during the 2020-21 season. Each week, more than 3,000 outpatient health care providers around the country report to CDC the number of patient visits for ILI by age group (0-4 years, 5-24 years, 25-49 years, 50-64 years, and ≥65 years) and the total number of visits for any reason. A subset of providers also reports total visits by age group. For this system, ILI is defined as fever (temperature of 100°F [37.8°C] or greater) and a cough and/or a sore throat. Since the 2021-22 season, the case definition no longer includes “without a known cause other than influenza”. Since ILINet monitors visits for ILI and not laboratory-confirmed influenza, it will capture visits due to any respiratory pathogen that presents with the symptoms of fever plus cough or sore throat. These data should be evaluated in the context of other surveillance data to obtain a complete and accurate picture of influenza activity.
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