This graph from the Georgia Division of Public Health (updated 6-25-2010) shows visits for influenza-like illness (ILI) in our state during week 24. The Georgia ILI curve began a sharp upward trajectory during the second week of August (see below), indicating the resurgence of  pandemic 2009 H1N1 when children began to return to school after summer vacation. The resurgence began in Georgia and then spread nationwide. The Georgia curve showed a steady decline from the first week of September until early January, when it leveled off. There was a regional persistence of flu circulation in the South East, including Georgia, that was noted from late January to late March 2010. The ILI rate in Georgia as of week 10 (March 2010) was 3.3%, and hospitalizations and deaths in Georgia due to H1N1 rose slightly during the months of February and March. ILI activity has subsequently declined in our state.  As of the end of June, only about 1% of emergency deparment visits in Georgia were for influenza-like illness (see explanation below). The second wave of pandemic 2009 H1N1 is over.

Note that H1N1 vaccine did not become available in Georgia or anywhere else in the United States until mid-October. This means the decline in ILI in Georgia began before the arrival of vaccine against H1N1. It is not clear how much of an impact the H1N1 vaccine had on the decline of flu cases in Georgia or in the nation. 

For the complete weekly influenza update from the Georgia Divison of Public Health please click here.

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. 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 an influenza outbreak. 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 that are not influenza. The summer of 2009 was a major exception to this rule, reflecting the spread of pandemic flu due to 2009 H1N1 during the US summer months. Georgia largley escaped the H1N1 outbreak during the first few months of this pandemic. The national curve showed much higher rates of ILI during June and July than did the Georgia curve.