Tracking Influenza-like Illness in Indiana

Indiana Epidemiology Newsletter
February 2007

Michael Wade, MPH
Syndromic Surveillance Epidemiologist

Shawn Richards, BS
Respiratory Epidemiologist

To better understand the types of influenza viruses present in the United States each year, the Centers for Disease Control and Prevention (CDC) coordinates a nationwide tracking program that depends on data submitted by individual states.  These data include the percentage of patients exhibiting influenza-like illness (ILI) and laboratory results of confirmed influenza cases.  The Indiana State Department of Health (ISDH) tracks influenza and ILI in two ways: 1) sentinel surveillance and 2) syndromic surveillance.  Due to heightened concern regarding the potential for pandemic influenza, influenza and ILI surveillance programs are essential. 

Sentinel Surveillance

Indiana participates in the U.S. Influenza Sentinel Provider Surveillance program coordinated by the CDC.  The ISDH has recruited 40 of the 1,000 health care providers around the country who report the percentage of patients with ILI seen in their offices each week on a year-round basis.  The CDC defines ILI for the purpose of surveillance as "fever (>100oF [37.8oC] oral or equivalent) and cough or sore throat (in absence of a known cause)."  Sentinel sites submit their data weekly to the repository at the CDC via Internet or fax.  Additionally, sentinel physicians collect nasopharyngeal swabs from random patients with ILI whose onset of classic clinical signs started within 72 hours of the appointment.  The swabs are sent to the ISDH Laboratories for viral isolation and identification to determine which influenza viruses are circulating. 

To facilitate the process, the ISDH provides sentinel sites with viral specimen submission kits, delivery of kits to the sentinel site, prepaid overnight shipping from the site to the ISDH Laboratory, periodic reports of influenza incidence in Indiana and the nation, educational opportunities regarding influenza and pandemic influenza, and a subscription to the Journal of Emerging Infectious Disease – all free of charge.  Sentinel sites that regularly report their data receive an official certificate from the CDC and the ISDH.  Physicians interested in serving in the sentinel surveillance program may contact Shawn Richards, Respiratory Epidemiologist, at 317.233.7125.

Syndromic Surveillance

The syndromic surveillance program in Indiana is known as the Public Health Emergency Surveillance System (PHESS).  The PHESS depends primarily on emergency department patient chief complaint data from 70 hospitals across the state to estimate the level of ILI present during the winter influenza season.  These chief complaint data are processed by a software application (ESSENCE) and are coded as ILI if they contain “influenza”, “flu”, or a combined complaint of “fever” and “cough” or “sore throat”.

Both the sentinel program and the PHESS report ILI as a percentage.  The number of patient visits resembling influenza serves as the numerator, and the total number of patient visits serves as the denominator (i.e., % ILI = (ILI pt visits / total pt visits) x 100).  While the sentinel system data typically yield a weekly ILI percentage approximately 1 percent greater than the PHESS, the two surveillance systems reflect remarkable agreement regarding time series trending.  The ISDH Respiratory Epidemiologist distributes weekly ILI percentages from sentinel sites and the PHESS, as well as data interpretation, to health care professionals in Indiana.

Influenza-Related Death Reporting

In addition to morbidity, mortality information from seasonal influenza is also very significant.  The Centers for Disease Control and Prevention (CDC) estimates that during 1990-1999, approximately 36,000 influenza-related deaths occurred each year in the United States (1).  Prior to October 2006, influenza death reports in Indiana were extracted exclusively from death certificates.  Because of the delay in filing death reports and the tracking method, numbers of influenza-related deaths in Indiana were delayed for up to 1½ years.  However, since October, 2006, all health care providers must report influenza-related deaths to the local health department within 72 hours of knowledge of the death.  This mandatory reporting allows for much more timely data collection and analysis regarding influenza-related deaths.


Thompson WW, Shay DK, Weintraub E, et al. Mortality associated with influenza and respiratory syncytial virus in the United States. JAMA 2003;289:179--86