National Visits Influenza-like Illnesses: March 1, 2020

March 1: CDC graph of visits nationally for Influenza-like illness (ILI), 2019-2020 in red triangles. The rest of the curves above represent data from prior years.

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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 viruses), Respiratory Syncitial 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 Spring, 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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Past Updates

Feb 22, 2020: There is a one-week lag between data collection and data availability through the CDC. The US peaked in late December with primarily influenza B, which has tapered off substantially. Around the first of the year, Influenza A was isolated more frequently, and flu A has now largely replaced flu B. It appears our peak of flu A happened nationally in mid-February. The flu season is not over, but it is winding down. Flu should be largely gone by mid- to late March.

February 10 CDC graph of visits nationally for Flu-like illness, red triangles. There is a one-week lag between data collection and data availability through the CDC. The US peaked in late December with primarily influenza B, which is now fading. However, influenza A is increasing dramatically. We are now experiencing a second peak of influenza which is likely to be at least as high in flu activity as our first peak in December.

January 25, 2020 CDC graph of visits nationally for Flu-like illness, red triangles. There is a one-week lag between data collection and data availability through the CDC. WE peaked in late December with primarily influenza B, which is now fading rapidly. However, influenza A is now increasing dramatically. We may be in for a double peak in flu activity this year.

December 28, 2019 The national graph of visits for flu-like illness appears to be peaking. Flu activity is high now throughout the US, with all regions except the upper plains states and North West states reporting widespread flu.

December 7, 2019: We have been above the epidemic threshold nationally for 4 weeks; we are in the flu season, although it is just beginning. It will get much more widespread and activity will increase.

November 23, 2019: This curve shows the rate of influenza-like illness in clinics nationally. The rate has just crossed the epidemic threshold, which is about 2.4%. This means that we have likely entered the seasonal flu epidemic in the United States. As can be seen by looking at data from previous years, this is a bit of an early start to the flu season.

November 15, 2019: This graph shows summary data on Influenza Like Illness (ILI) over the past several seasons, with the current season represented by red triangles in the lower lower left corner. As of November 2, 2019, flu activity is very low in the United States but is increasing toward the epidemic threshold of 2.4%. Flu season is coming to the US, and will likely show substantial activity in the next 4-6 weeks