National Visits Influenza-like Illnesses: July 2019 

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This graph shows summary data on Influenza Like Illness (ILI) over the past several seasons. Currently flu activity is very low in the United States.

  Click here to get more detailed information from CDC.

How is this curve constructed?       Each week a group of “sentinel providers” around the country reports to CDC on how many of their patients in emergency departments and clinics have illnesses that match the CDC case definition for “influenza-like illness”, or ILI. ILI is defined as fever PLUS either cough or sore throat that cannot be attributed to another cause such as Strep throat (i.e. Strep throat is NOT an ILI). This data is collected at CDC and posted.

What does the curve mean?     The ILI curve does not tell us the total number of people with flu, it tells us what proportion of patients around the country have an illness like the flu. ILI occurs at a low background rate of 1-2%  throughout the year, even when flu is not circulating.  This background rate of ILI is due to other viruses such as  Rhinovirus (cold virus), RSV, and Parainfluenza. Years of experience has shown that when the ILI curve rises above its baseline and peaks 4-8 weeks later, it closely correlates with actual influenza activity as determined by more tedious and expensive viral cultures. Thus, the ILI curve provides a convenient way to track the progress of our yearly influenza season, which almost always arrives between December and March. During the winter (i.e., flu season), most people with ILI actually do have the flu. During the summer and autumn months, patients with ILI almost invariably are infected with other respiratory viruses. The summer of 2009 was a major exception to this rule, reflecting the first wave of pandemic flu due to 2009 H1N1 during the US summer months.

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