This graph posted by the Centers for Disease Control and Prevention (CDC) on May 28 2010 shows visits for influenza-like illness (ILI) nationwide during the week ending May 22. There is generally a  one-week lag between data collection and release to the public. The units at the bottom of the graph indicate the date by week of the year. Flu activity has abated signficantly since the peak  in October 2009. The current curve shows ILI activity nationally has now been below the epidemic threshold since the beginning of the year. The persistence of circulating influenza in the US Southest that was noted from February to April 2010 has  abated.

Note that the ILI curve has declined substantially, well below the epidemic threshold, and is now close to the baseline of 1%. See “What does this curve mean” (below) for an explanation.

The ILI curve began an upward trajectory during the second week of August 2009, corresponding to the “second wave” of the   2009 H1N1 pandemic when children began to return to school after summer vacation. This resurgence began in Georgia and then spread nationwide. It is interesting to note that the US Southeast, including Georgia, was also the last significant region of influenza activity in the pandemic’s second wave.

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 peaks in the winter, 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 H1N1 during the US summer months.